Skip to main content

Full text of "Pioneering Venus : a planet unveiled"

See other formats



A  Planet  Unveiled 

From  the  collection  of  the 



xy    2 

o  Prejinger 
v    Jjibrary 


San  Francisco,  California 


?EB  1  6  |S9! 


A  Planet  Unveiled 

The  Pioneer  Project  and  the 
Exploration  of  the  Planet  Venus 

Library  of  Congress  Cataloging-in-Publ ication   Data 

Fimmel ,    Richard  0. 

Pioneering  venus    :    a   planet   unveiled   /   Richard  0.    Fimmel,    Lawrence 
Colin,    Eric  Burgess    ;    prepared  at   NASA  Ames   Research   Center. 

p.      cm. 

Includes  bibliographical  references  and  index. 
ISBN  0-9645537-0-8  (hardcover).  --ISBN  0-9645537-1-6  (pbk.) 
1.  Venus  probes.  2.  Pioneer  (Space  probes)  I.  Colin,  Lawrence. 
II.  Burgess,  Eric.   III.  Title. 
QB621.F55  1995 

523.4'2--dc20  95-16551 


For  Sale  by  the  Superintendent  of  Documents 

U.S.  Government  Printing  Office,  Washington,  D.C.  20402 


V  '  E  '  N  '  U  -  S 

A  Planet  Unveiled 

Richard  O.  Fimmel 

Manager,  Pioneer  Missions 
Ames  Research  Center 

Lawrence  Colin 

Project  Scientist 

Eric  Burgess 

Science  Writer 

Prepared  at  NASA  Ames  Research  Center 

National  Aeronautics  and 
Space  Administration 













8    VENUS  AND  EARTH:   * 







Since  their  first  launches  in  1958,  pioneering  space- 
craft have  made  many  trail-blazing  discoveries  in 
exploring  the  Solar  System.  Initial  attempts  of  our 
nation  to  probe  interplanetary  space  were  with 
spacecraft  named  Pioneer.  The  first  of  several  small 
spacecraft  was  launched  successfully  by  the  Air  Force 
Ballistic  Missile  Division  in  November  1958.  One  of 
these  spacecraft  escaped  completely  from  Earth's 
gravity  and  went  into  solar  orbit.  It  was  the  first 
interplanetary  spacecraft. 

Subsequently,  NASA's  Pioneers  6  through  9  made 
major  discoveries  about  interplanetary  particles  and 
fields  and  the  solar  wind.  Pioneers  10  and  1 1  explored 
the  asteroid  belt  and  the  magnetospheres  and  physical 
natures  of  the  giant  outer  planets  Jupiter  and  Saturn. 
They  discovered  a  new  ring  and  new  satellites  of 
Saturn  and  sent  to  Earth  the  first  images  from 
spacecraft  of  the  Galilean  satellites.  Pioneer  10 
showed  that  spacecraft  could  safely  travel  through 
the  asteroid  belt  and  survive  passage  through  the 
intense  radiation  environment  of  Jupiter,  paving 
the  way  for  the  Pioneer  1 1  mission  to  Saturn. 
After  their  planetary  encounters,  Pioneers  10 
and  1 1  headed  out  from  the  Solar  System  to 
the  distant  stars. 

Pioneer  Venus  carried  on  the  pioneering 
tradition.  The  two  spacecraft  of  the 
mission,  an  Orbiter  and  a  Multiprobe 
Bus,  were  launched  from  Kennedy  Space 
Center  in  1978.  Four  probes  and  the  Multiprobe  Bus 
penetrated  the  atmosphere  of  Venus  and  gathered 
important  new  data  from  the  exosphere  to  the  planet's 
hot  surface.  This  probe  gave  scientists  new  insights 
not  only  about  the  Venusian  atmosphere,  but  also 
about  planetary  atmospheres  in  general. 

The  14-year  Orbiter  mission  was  equally  successful. 
Its  payload  of  advanced  science  instruments  gathered 
a  wealth  of  data  about  the  atmosphere  and  ionosphere 
of  Venus  and  their  interactions  with  the  solar  wind. 
Additionally,  the  instruments  penetrated  the  dense 

clouds  of  Venus  for  the  first  time  and  revealed  global 
details  of  the  planet's  intriguing  surface. 

Pioneer  Venus  yielded  a  high  scientific  return  for  a 
relatively  low  cost.  It  produced  valuable  scientific  data 
from  1978  through  1992 — over  more  than  one 
complete  solar  activity  cycle.  For  14  years  the 
spacecraft  continued  in  excellent  working  order.  Its 
conservative  design  maintained  all  functions  with  only 
modest  reductions  from  their  original  performance  at 
the  beginning  of  the  mission.  No  complete  failure  of 
any  critical  component  occurred. 

The  Pioneer  Venus  program  was  remarkable  in  the 
way  it  successfully  pursued  investigations  over  a 
broad  range  of  planetary  sciences.  These  included 
information  gathered  by  the  probes,  radar  and  gravity 
mapping  of  the  surface  of  Venus,  investigations  of  the 
atmosphere  from  the  surface  through  the  clouds  and 
the  ionosphere  to  the  exosphere,  and  the  interaction  of 
Venus  with  solar  wind. 

Also  of  great  importance  was  the  way  the  Pioneer 
Venus  program  created  a  sense  of  collegia!  scientific 
investigators  cooperating  with  engineers  and  other 
mission  personnel  as  a  highly  effective  team  for  nearly 
20  years  of  planning  and  mission  operations. 

Although  the  Orbiter's  final  entry  into  the  Venusian 
atmosphere  in  October  1 992  ended  Pioneer  Venus 
operations,  the  mission  continues  as  scientists  access 
its  extensive  archives  of  data  about  Venus.  The 
mission  also  provided  important  groundwork  for 
NASA's  highly  successful  Magellan  mission  to 
Venus.  Undoubtedly,  the  experience  gained  from 
Pioneer  Venus  will  continue  to  be  of  great  value  to 
planners  of  future  missions  to  the  strange 
twin  of  Earth. 

Ken  K.  Munechika 

Director,  NASA  Ames  Research  Center 



Pioneer  Venus  Orbiter  completed  an  unprecedented 
14  years  in  orbit  about  Venus,  from  December  1978  to 
October  1992.  In  this  NASA  Special  Publication  we 
describe  for  a  wide  readership  the  scientific  discover- 
ies not  only  of  the  Orbiter  but  also  of  the  four  probes 
and  the  Multiprobe  Bus  that  entered  the  atmosphere  of 
Venus  and  made  many  scientific  measurements  within 
that  atmosphere. 

The  great  excitement  of  any  age  has  been  created  by 
pioneers — those  who  sought  out  new  lands,  new 
ideas,  new  social  systems,  new  forms  of  governance 
and  new  goals  for  humankind.  In  our  time  we  have 
been  privileged  to  witness  and  be  part  of  an  outstand- 
ing human  achievment  of  pioneers  probing  a  great  new 
frontier,  space.  Space  pioneering  has  been  a  team 
effort  of  many  people;  dreamers,  planners,  technicians, 
engineers,  scientists,  and  managers.  The  objective  has 
been  to  broaden  human  knowledge  about  the  wider 
environment  beyond  Earth  and  how  this  environment 
affects  our  own  planet.  To  this  end  NASA  sought 
information  about  the  other  planets  of  the  Solar 
System  through  a  series  of  interplanetary  missions. 
Of  these  pioneering  missions,  the  one  described  in  this 
book  targeted  cloud-shrouded  Venus  which  in  several 
ways  seemed  to  be  a  twin  of  Earth,  but  in  others  quite 
different  from  our  planet.  Scientists  wanted  to  know 
how  and  why  it  differed. 

The  Pioneer  Venus  mission  studied  practically  all 
aspects  of  the  environment  of  Venus.  Scientific  in- 
vestigations covered  surface  geology  and  electrical 
properties,  gravity  field,  intrinsic  and  induced  magnetic 
fields,  neutral  atmosphere  composition  and  tempera- 
ture structure,  cloud  structure  and  microphysics,  at- 
mospheric electrical  discharges,  ionospheric  composi- 
tion and  temperature  structure,  and  the  complicated 
physics  of  the  interaction  of  the  solar  wind  with  the 
planet  over  more  than  one  solar  activity  cycle. 

Many  space  scientists  devoted  a  major  part  of  their 
professional  careers  to  this  mission.  They  published  a 
wealth  of  scientific  papers;  well  over  1000  in  a  wide 
range  of  science  journals.  Of  these  Pioneer  Venus 

scientists,  45%  were  from  institutions  in  academia, 
47%  from  federal  laboratories,  and  8%  from  industrial 
laboratories.  Thirty-four  colleges  and  universities, 
14  federal  laboratories,  and  15  industrial  laboratories 
were  involved,  and  ten  countries  outside  the  U.S. 
were  represented. 

The  mission  demonstrated  how  a  large  amount  of 
scientifically  important  information  can  be  obtained 
in  an  extremely  efficient  and  cost  effective  manner. 
A  small  number  of  management  and  spacecraft 
operations  personnel  supported  the  Pioneer  Venus 
mission,  relative  to  the  large  number  of  benefitting 
scientists.  The  mission  was  one  of  the  most  scientifi- 
cally beneficial,  low-cost  programs  conducted  by 
NASA.  It  benefitted  from  a  "lean  and  mean"  highly 
professional  project  management  and  operations 
organization.  The  average  annual  funding  to  operate 
the  mission  over  13  years  was  $5  million,  of  which 
60%  was  spent  on  science  and  40%  on  management 
and  operations.  These  laudable  results  were  obtained 
by  a  team  of  dedicated,  hardworking  engineers  and 
scientists  who  never  underestimated  the  value  of 
what  they  were  doing. 

In  preparing  this  final  report  about  the  pioneering 
mission  to  our  neighbor  planet,  we  set  out  to  make 
the  presentation  of  information  suitable  for  a  wide 
readership  including  current  and  future  students. 
Toward  this  end  we  appreciated  the  work  of  John 
Boeschen,  our  editor,  who  helped  us  simplify  much 
of  the  involved  science  and  technical  material. 

We  are  grateful  to  R.  Z.  Sagdeev,  V.  I.  Moroz, 
and  T.  Breus  who  supplied  the  material  for  Chapter  7 
about  Soviet  missions  to  Venus.  Also,  we  thank 
T.  M.  Donahue  for  contributing  the  important  final 
chapter  in  which  he  points  out  the  relevance  of 
studies  of  Venus  to  improving  our  understanding  of 
Earth's  evolution. 

Richard  O.  Fimmel 

Lawrence  Colin 

Eric  Burgess  July  1994 


'•'    * 

V   '  1 



To  the  Memory  of: 


Original  Principal  Investigator, 

Large  Probe  Infrared  Radiometer. 

Interdisciplinary  Scientist, 
Radar  team. 



.  Original  Principal  Investigator, 
Orbiter  Electric  Field  Detector. 

.  Original  Principal  Investigator, 
Solar  Wind  Plasma  Experiment. 

Pioneer  Venus  team  members  and 



Mariner  1 0  flies  by  Venus  above 
the  cloud  shrouded  planet. 


Jet.  Public  Library 
FEB  1  6 

This  special  publication  presents  the  exciting 
story  of  Pioneer  Venus,  a  National  Aeronautics 
and  Space  Administration  (NASA)  program.  In 
the  following  pages,  you  track  the  mission 
from  its  start  through  its  highly  successful 
operations  and  conclusion.  This  chapter  flips 
back  the  calendar  to  the  late  1960s  when 
initial  planning  for  an  in-depth  exploration  of 
Venus  began. 

You  might  wonder  what  it  was  about  Venus 
that  rallied  thousands  of  scientists  behind 
Pioneer  Venus.  To  understand  their  enthusi- 
asm for  Earth's  sister  planet,  a  review  of  what 
we  knew  about  Venus  before  Pioneer  is 
important.  Pioneer  Venus  gives  you  this  review. 
It  also  describes  intriguing  new  knowledge  that 
earlier  U.S.  and  U.S.S.R.  spacecraft  missions 
brought  us  and  why  those  missions 
emphasized  the  need  for  a  Pioneer  mission. 
Later  chapters  describe  the  mission's  space- 
craft, experiments,  results,  and  their  implica- 
tions, and  then  provide  background  informa- 
tion about  related  Soviet/Russian  missions. 

^Pre-Space-Age  Knowledge 

kThe  brilliant  planet  Venus  has  intrigued 
humans  since  ancient  times.  The 
highly  reflecting,  cloud-shrouded 

planet  is  clearly  visible  from  Earth, 
and  shines  brighter  than  all  other 
objects  in  the  sky  except  the  Sun  and 
Moon.  Its  risings  and  settings  have  been 
noted  in  many  ancient  records,  including 
Babylonian  clay  tablets  and  Mayan  codices. 
However,  our  ancestors  did  not  understand 
these  motions  until  the  15th  century.  At  that 
time,  the  Copernican  revolution  in  human 
thought  acknowledged  the  Sun,  not  the  Earth, 
to  be  the  center  of  the  Solar  System,  and  that 
all  the  planets,  including  the  Earth  and  Venus, 
revolve  around  it.  The  coming  of  the  telescope 

in  the  1 7th  century  revealed  Venus  as  more 
than  a  star-like  point  of  light.  Now  astrono- 
mers could  measure  the  planet's  apparent 
angular  diameter  and  study  its  moon-like 
phases.  These  phases  result  from  Venus' 
having  an  orbit  that  is  inside  that  of  the  Earth. 
With  a  good  pair  of  field  glasses,  you  can  see 
these  phases  yourself. 

Venus  is  the  one  planet  in  our  Solar  System 
most  similar  to  Earth  in  size  and  mass.  Venus' 
mass,  diameter,  and  density  are  all  only 
slightly  less  than  Earth's.  There  the  resem- 
blance ends.  Its  atmosphere  is  100  times  as 
dense  as  Earth's.  Its  surface  is  hot  enough  to 
melt  lead.  It  rotates  very  slowly  on  its  axis  and 
has  virtually  no  water.  Its  dense  atmosphere 
consists  mainly  of  carbon  dioxide  with  clouds 
of  sulfuric  acid  droplets.  These  differences 
intrigued  planetary  scientists,  and  they 
wondered  why  the  two  planets  evolved  along 
such  different  paths.  Why  is  one  capable  of 
supporting  life  but  not  the  other? 

The  image  of  Venus  as  seen  through  the  best 
telescope  is  brilliant  but  uninteresting,  and 
reveals  little  detail.  During  a  relatively  brief 
period  in  history,  astronomers  tried  to  measure 
the  planet's  rotation  period  and  searched  for 
some  satellites,  or  moons,  but  they  failed.  Not 
too  surprisingly,  they  shifted  their  interest  to 
other,  more  revealing  objects  in  the 
Solar  System. 

Eventually  the  development  of  new  tech- 
niques spurred  a  revival  of  interest  in  Venus 
research.  Beginning  in  the  early  1900s, 
photographic  and  other  instruments  were 
developed  along  with  powerful  analytic 
methods.  These  could  then  be  used  to  study 
Venus  over  a  wide  range  of  the  electromag- 
netic spectrum. 

Venus  is  visible  from  Earth, 
and  humans  have  been 
observing  the  planet  for 
thousands  of  years.  The  first 
sections  in  this  chapter 
review  our  knowledge  of 
Venus  up  to,  but  not  includ- 
ing, Pioneer  Venus.  In  these 
early  pages,  you  learn  about 
Venus'  physical  features  and 
its  place  in  our  Solar  System. 
The  chapter  concludes  with 
descriptions  of  pre-Pioneer 
Venus  space  missions,  their 
discoveries,  and  questions 
they  left  unanswered  about 
the  planet. 

Figure  1-1 .  Because  Venus 
orbits  the  Sun  within  Earth's 
orbit,  it  appears  to  stay  close 
to  the  Sun  as  we  observe  it 
from  Earth.  At  its  greatest 
angular  distance  from  the 
Sun,  Venus  is  at  eastern  or 
western  elongation.  In  this 
figure,  the  planet  appears  at 
eastern  elongation  when  it 
sets  after  the  Sun,  and  we 
see  it  as  an  evening  "star. " 
At  western  elongation,  it  is 
visible  rising  before  the  Sun 
as  a  morning  "star. " 

Scientists  used  infrared  wavelengths  to  charac- 
terize the  clouds  and  overlying  atmospheric 
gases.  Information  about  the  surface  and  lower 
atmosphere  came  from  microwave  emissions. 
Analysis  of  radar  signals  that  bounced  off  the 
planet  determined  its  period  of  rotation.  How- 
ever, major  discoveries  about  Venus  had  to 
wait  until  the  1960s  when  spacecraft  became 
available  to  explore  the  planet.  The  first  suc- 
cessful interplanetary  probe,  Mariner  2,  flew 
by  Venus  in  1962.  That  flight  began  the  space- 
age  exploration  of  the  second  planet  and  our 
Solar  System. 

Path  of  Venus 
in  orbit 

Apparent       \ 
daily  path  of 
setting  sun 

Venus  near 


Horizon         conjunction 

Venus  near 
O  conjunction 

Sun  below 


Venus  as  a  Member  of  the 
Solar  System 

Astronomers  call  Venus  an  inferior  planet 
because  it  revolves  around  the  Sun  inside 
Earth's  orbit.  (Its  average  distance  from  the 
Sun  is  72.3%  of  Earth's  average  distance  from 
the  Sun).  As  a  result,  you  see  Venus  as  either  a 
morning  or  an  evening  "star."  Early  peoples 
believed  these  two  bright  "wandering  stars" 
were  separate  objects  and  gave  them  different 

names.  The  Greeks,  for  example,  named  them 
Phosphorus  and  Hesperus. 

Venus  appears  to  move  through  the  constella- 
tions of  the  zodiac.  It  travels  close  to  the 
ecliptic— the  apparent  yearly  path  of  the  Sun 
relative  to  the  stars,  which  is  the  plane  of 
Earth's  orbit  projected  against  the  stars — and 
oscillates  east  and  west  of  the  Sun  but  never 
more  than  48  degrees  from  it.  We  call  the 
planet's  positions  at  maximum  angular  dis- 
tance east  and  west  of  the  Sun  the  eastern  and 
western  elongations,  respectively.  At  eastern 
elongation,  Venus  is  an  evening  object. 
Each  day,  it  follows  the  Sun  across  the  sky 
(Figure  1-1).  At  western  elongation,  Venus  rises 
before  the  Sun  each  day.  The  planet  passes 
from  greatest  eastern  elongation  to  greatest 
western  elongation  in  about  144  days  and 
from  western  to  eastern  in  about  440  days. 

Because  it  reflects  71%  of  the  sunlight  that 
bathes  it,  Venus  is  bright  enough  to  see  at 
midday  if  you  know  where  to  look.  It  is 
brightest  about  one  month  before  and  one 
month  after  inferior  conjunction.  This  is  when 
the  planet  passes  closest  to  Earth  between 
Earth  and  Sun.  As  noted  earlier,  Venus  exhibits 
phases  like  the  Moon  (Figure  1-2).  When  it  is 
brightest  in  Earth's  skies,  Venus  appears  as  a 
fat  crescent. 

Venus  takes  224.7  days  to  revolve  around  the 
Sun  in  its  almost  circular  orbit  (the  orbit  has  a 
mean  radius  of  108.2  million  km,  or  67.2  mil- 
lion miles).  Because  Earth  also  moves  around 
the  Sun,  the  periods  when  Venus  is  visible  at 
elongations  or  at  conjunctions  repeat  every 
583.92  days.  Opportunities  to  send  spacecraft 
to  Venus  with  minimum  energy  also  repeat 
with  this  period. 

When  behind  the  Sun  at  superior  conjunction, 
Venus  is  257.3  million  km  (159.9  million 

1910  SEPT  27 

1910  JUNE  10 

1927  OCT  24 

Figure  1  -2.  Galileo  discovered 
that  Venus,  seen  through  a 
telescope,  shows  phases  similar 
to  the  Moon 's.  These  photo- 
graphs from  Lowell  Observatory 
show  the  phases  and  how  the 
planet  looks  much  larger  in  the 
crescent  phase  as  it  comes 
between  the  Earth  and  Sun  at 
inferior  conjunction.  You  can  see 
this  crescent  shape  with  the  aid 
of  a  good  pair  of  field  glasses. 

(Appendix  A  lists  some  major  events 
in  the  exploration  of  Venus  by 
Earth-based  observations  and  from 
theoretical  inferences.) 

miles)  from  Earth.  At  inferior  conjunction 
(Figure  1-3),  Venus  is  41.9  million  km  (26  mil- 
lion miles)  from  Earth.  However,  Earth's  orbit 
is  inclined  3.4  degrees  to  Venus'  orbit,  so 
Venus  is  nearly  always  slightly  above  or  below 
the  Sun  at  inferior  conjunction.  Only  infre- 
quently does  the  planet  travel  in  front  of  the 
Sun  (as  we  see  it  from  Earth).  Scientists  refer  to 
this  movement  as  a  transit.  During  transit, 
Venus  is  visible  as  a  small  black  disk  silhou- 
etted on  the  bright  face  of  the  Sun.  Transits  of 
Venus  occur  in  pairs  8  years  apart  with  over  a 
century  intervening  between  successive  pairs. 
The  most  recent  transits  occurred  in  1874  and 
1882,  and  the  next  pair  will  be  on  June  7, 
2004,  and  June  5,  2012. 

,'  Venus          ^v 

--^--  N 

x  ^ — '        \  \ 

'  /         Superior         \  \ 

.  /         conjunction         \  \ 

/          /  \         N 

/    /        ^fc        \    \ 

I          I 

\  4) 


v  Inferior  /' 

\  \       conjunction 

\  ^ 



Figure  1-3.  When  Venus  is 
closest  to  Earth  at  inferior 
conjunction,  the  planet  is 
between  the  Earth  and  Sun. 
On  the  far  side  of  the  Sun, 
and  most  distant  from  Earth, 
Venus  is  at  superior  conjunc- 
tion. Sometimes  at  inferior 
conjunction  the  positions  of 
Venus  and  Earth  on  their 
orbits  are  such  that  Venus 
passes  in  front  of  the  Sun's 
disc.  Astronomers  call  this 
passage  a  transit.  The  next 
pair  of  transits  occurs  early  in 
the  2 1st  century. 

Figure  1-4.  When  Venus  transits 
the  Sun's  disk,  the  planet's 
atmosphere  distorts  the  black 
spot  silhouetted  against  the 
bright  solar  photosphere.  This 
optical  effect  reveals  that  Venus 
has  an  atmosphere.  Also,  when 
Venus  is  dose  to  the  Sun,  as 
you  observe  it  from  Earth,  its 
bright,  thin  crescent  extends 
around  the  dark  globe.  This 
occurs  because  of  the  effects 
of  the  planet's  atmosphere 
(see  the  leftmost  diagram). 

In  the  past,  astronomers  used  Venus  transit 
times  to  help  determine  the  Earth's  distance 
from  the  Sun.  In  1874  and  1892,  astronomers 
therefore  journeyed  to  remote  regions  of  the 
globe  to  observe  Venus'  transit  with  sensitive 
instruments.  However,  their  efforts  were  foiled 
by  a  strange  optical  effect  (Figure  1-4).  As 
transit  started,  the  planet's  black  disk  would 

Venus  as  a  Planet 

Why  is  Venus  so  different  from  Earth?  The 
environment  on  Venus  today  differs  signifi- 
cantly from  our  planet's.  Its  surface  is  much 
hotter,  and  its  atmosphere  is  nearly  100  times 
as  dense.  Also,  its  rotation  is  much  slower  and 
is  retrograde,  meaning  it  is  in  the  direction 
opposite  to  Earth's  rotation  and  the  general 

Table  1-1.  Orbit  of  Venus 

Mean  distance  from  Sun 

0.723  AU 
108.2  million  km 
67.2  million  miles 

Inclination  of  orbit  to  plane  of  the  ecliptic 


Sidereal  period  (period  with  respect  to  the  stars) 

224.7  Earth  days 

Mean  synodic  period  (period  with  respect  to  Earth) 

583.92  Earth  days 

Mean  orbital  velocity 

35.05  km/sec 
21.78  miles/sec 

Closest  approach  to  Earth 

41.9  million  km 
26.0  million  miles 

appear  to  remain  connected  to  the  dark  sky 
beyond  the  limb  of  the  Sun  (i.e.,  the  edge  of 
the  Sun's  optical  disk).  The  connection  would 
thin  to  a  mere  thread,  then  snap.  The  Russian 
chemist  M.  V.  Lomonosov  transformed  this 
annoying  effect  into  an  important  discovery 
when  he  correctly  attributed  it  to  an  atmo- 
sphere around  Venus. 

Table  1-1  summarizes  the  characteristics  of 
Venus'  orbit. 

motions  of  the  planets  around  the  Sun.  To 
make  observation  even  more  difficult,  unbro- 
ken, planet-wide  clouds  hide  Venus'  surface.  In 
ultraviolet  light,  these  clouds  show  markings 
that  appear  to  rotate  about  the  planet  in  a 
period  of  4-5  days.  Astronomers  had  discov- 
ered that  the  mainly  carbon-dioxide  atmo- 
sphere contained  only  minute  amounts  of 
water  vapor.  Because  Venus'  magnetic  field  (if 
there  is  one)  is  small,  the  planet's  interaction 
with  the  solar  wind  is  different  from  Earth's. 

Table  1-2.  Physical  Data  on  Venus 

Diameter  (solid  surface) 

7,51 9  miles 

0.95  Earth's  diameter 

Diameter  (top  of  clouds) 

1 2,240  km 
7,606  miles 


48.8  X  1026g 
0.815  Earth  masses 


5.269  gm/cm3 
0.96  Earth's  density 

Axial  rotation  period  (retrograde) 

243.1  Earth  days 

Rotation  period,  cloud  tops  (retrograde) 

4.0  Earth  days  (approximately) 

Period  of  solar  day 

1 1 6.8  Earth  days 

Inclination  of  rotation  axis 


Surface  gravity 

888  cm/sec2 
0.907  g 

Surface  atmospheric  pressure 

9,61 6  kPa 

95  Earth  atmospheres 

Surface  temperature 

750  K  (approximate) 
480°C  (approximate) 
900°F  (approximate) 

Reflecting  capability  (albedo) 


1 .82  Earth's  albedo 

Stellar  magnitude  when  brightest 


Venus  also  lacks  a  satellite.  Physical  data  on 
the  planet  appear  in  Table  1-2. 

Period  of  Rotation 

Look  through  an  optical  telescope  on  Earth. 
Try  as  you  might,  you  won't  see  clear  details 
on  Venus'  brilliant,  yellowish  disk.  Some  early 
observers,  though,  claimed  they  saw  faint, 
elusive  markings.  Did  they  really  see  them?  We 
can't  be  sure.  However,  the  markings  they 
described  were  similar  to  those  you  would 
expect  on  extensive  cloud  systems. 

As  late  as  1964,  Earl  C.  Slipher,  famous  plan- 
etary photographer  of  Lowell  Observatory, 
Flagstaff,  Arizona,  wrote,  "All  the  early  efforts 
to  photograph  Venus  at  Flagstaff  (from  1904 
on) . . .  succeeded  in  registering  only  faint 
vague  markings,  too  weak  to  add  new  informa- 
tion." The  general  absence  of  visible  surface 
features  prevented  astronomers  from  measur- 
ing Venus'  period  of  rotation.  Wildly  varying 
periods  were  claimed — from  24  Earth  hours 
to  a  period  equal  to  the  Venus  year 
(224.7  Earth  days). 

On  May  10,  1961,  a  radar  signal  from  a  NASA 
Deep  Space  Network  antenna  at  Goldstone, 
California,  was  bounced  off  Venus.  Analysis  of 
the  returned  echo  indicated  that  the  planet 
rotated  extremely  slowly.  Later,  radar  astrono- 
mers determined  that  Venus  rotates  about  its 
axis  in  243.1  Earth  days  in  the  direction 
opposite  to  Earth's.  Because  its  axial  rotation 
and  orbital  revolution  are  of  comparable 
periods,  a  solar  day  on  Venus  is  116.8  Earth 
days.  Just  imagine:  58  Earth  days  of  daytime 
and  an  equally  long  nighttime.  And  the  Sun 
rises  in  the  west  and  sets  in  the  east! 

Strangely,  Venus'  period  of  rotation  is  almost 
locked  to  the  periods  of  revolution  of  Earth 
and  Venus  around  the  Sun.  The  result:  Venus 
turns  very  nearly  the  same  hemisphere  to 
Earth  each  time  the  planet  passes  between 
Earth  and  Sun  at  inferior  conjunction. 

Why  Venus  rotates  so  slowly  is  still  an 
unsolved  mystery — most  other  planets  rotate 
in  periods  of  hours  rather  than  days.  While 
scientists  attribute  Mercury's  slow  rotation  to 

Figure  7  -5.  While  it  is  impos- 
sible to  see  through  the  clouds 
of  Venus  at  optical  wave- 
lengths, radar  can  penetrate 
to  the  surface.  Radar  maps 
of  Venus  show  many  surface 
features.  An  early  radar  picture 
of  the  planet's  surface  appears 
in  this  figure.  It  is  one  of  a 
series  that  R.  M.  Goldstein  of 
the  jet  Propulsion  Laboratory 
obtained  (he  used  equipment 
from  NASA  Deep  Space  Net- 
work with  a  large  antenna  at 
Coldstone,  California). 

the  Sun's  tidal  effects,  Venus  is  too  far  from 
the  Sun  for  such  effects  to  be  significant  over 
the  planet's  4.6  billion  year  lifetime.  One 
speculation  is  that  a  grazing  collision  with  an 
asteroid-sized  body  slowed  Venus'  rotation. 

Shape  of  Venus 

Scientists  have  used  the  Earth's  moon  and 
artificial  Earth  satellites  to  explore  its  gravita- 
tional field.  This  information,  along  with  the 
Earth's  deviation  from  perfect  sphericity  (i.e., 
its  oblateness),  can  be  used  to  help  develop 
models  of  the  Earth's  interior.  However,  Venus 
is  almost  a  perfect  sphere.  Its  lack  of  oblateness 
and  lack  of  a  satellite  prevented  astronomers 
from  developing  good  models  of  the  planet's 
internal  structure  and  composition.  Most 
planetologists  assumed  that  Venus'  interior 
was  similar  to  Earth's:  a  liquid  core,  a  solid 
mantle,  and  a  solid  crust. 

Surface  Features 

Venus'  surface  remained  a  mystery  until  radar 
probed  through  its  dense  atmosphere.  Using 
radar,  scientists  discovered  large  but  shallow 
circular  features  in  its  equatorial  regions. 

Scientists  believed  these  were  most  likely 
craters.  Stretching  1000  km  (621  miles)  north 
and  south  across  the  equator  (Figure  1-5)  was  a 
major  chasm.  Radar  observations  also  showed  a 
large-scale  granular  surface  structure,  which 
might  be  a  rock-strewn  desert.  Planetologists 
interpreted  some  areas  of  high  radar  reflectiv- 
ity as  vast  lava  flows  and  mountainous  areas. 

Despite  Venus'  dense  atmosphere  and  clouds, 
some  sunlight  does  penetrate  to  the  surface.  At 
these  locations,  solar  flux  is  about  equal  to  an 
overcast  day  in  midlatitudes  at  Earth's  surface. 
Instruments  measured  the  amount  of  solar 
radiation  at  the  surface  at  an  integrated  flux  of 
about  14,000  lux  (when  the  Sun  was  at  about  a 
30°  angle  from  overhead).  Photographs  from 
one  Soviet  lander  spacecraft  (Figure  1-6)  con- 
firmed a  dry,  rocky  surface  that  unknown 
processes  have  fractured  and  moved  about. 
A  second  lander  produced  a  picture  of  rocks 
with  rounded  edges  and  pitted  surfaces. 
Measurements  from  the  spacecraft  indicated 
that  surface  rocks  have  a  density  between 
2.7  and  2.9  g/cm3,  typical  of  basaltic  rocks 
on  Earth.  This  information  supported  earlier 
theories  that  Venus  had  separated  into  a  core, 
mantle,  and  crust. 

Other  early  spacecraft  results  showed  that 
Venus  had  little  water.  Did  Venus  ever  have 
oceans?  If  it  did,  what  happened  to  them? 
Some  researchers  speculated  that  water  rose  as 
vapor  into  the  high  atmosphere,  where  solar 
radiation  broke  it  down  into  hydrogen  and 
oxygen.  The  hydrogen  then  escaped  into  space 
from  the  top  of  Venus'  atmosphere  while 
heavier  oxygen  remained  and  oxidized  crustal 
rocks.  Others  hypothesized  that  Venus  might 
have  formed  so  close  to  the  Sun  that  high 
temperatures  within  the  solar  nebula  prevented 
water  from  condensing  and  becoming  part  of 
the  planet.  If  so,  Venus  would  never  have  had 
enough  water  within  its  rocks  to  form  early, 

deep  oceans  like  Earth's.  Our  oceans  played  a 
role  in  clearing  the  atmosphere  of  most  of 
Earth's  carbon  dioxide  through  the  reaction  of 
the  carbon  dioxide  with  water  to  form  carbon- 
ate rock.  By  contrast,  Venus'  carbon  dioxide 
has  remained  mainly  in  its  atmosphere. 

On  Venus,  because  of  high  surface  tempera- 
tures, scientists  expected  chemical  reactions 
between  the  atmosphere  and  the  minerals  in 
rocks  to  occur  much  faster  than  on  Earth. 
However,  on  our  wet  planet,  running  water 
continually  exposes  rocks  to  the  atmosphere 
and  speeds  chemical  reactions.  But  without 
water,  it  seemed  unlikely  that  such  processes 
would  take  place.  Of  course,  unless  fresh  rocks 
were  continually  exposed,  Venus'  atmosphere 
would  never  achieve  equilibrium  with  surface 

Atmospheric  Composition 

Although  astronomers  discovered  Venus' 
atmosphere  in  the  16th  century,  its  extent 
and  composition  remained  a  mystery  until 
recently.  The  planet's  atmosphere  consists  of 
three  distinct  regions:  the  part  above  the 
visible  cloud  tops,  consisting  of  the  ionosphere 
and  exosphere;  the  clouds;  and  the  region 
extending  from  the  base  of  the  clouds  to 
the  surface. 

In  the  1930s,  infrared  spectros- 
copy  revealed  carbon  dioxide 
absorption  bands  in  Venus' 
spectrum.  Carbon 
dioxide  appeared  to 
be  much  more 
abundant  in 

atmosphere  than  in  Earth's.  Later,  high- 
resolution  spectroscopy  confirmed  that  carbon 
dioxide  is  the  dominant  gas.  It  also  found 
traces  of  water  vapor,  carbon  monoxide, 
hydrochloric  acid,  and  hydrogen  fluoride. 
Unfortunately,  spectroscopy  could  not  reveal 
the  exact  amount  of  carbon  dioxide. 

Soviet  space  probes  that  penetrated  the  Venu- 
sian  atmosphere  (see  Chapter  7)  confirmed 
Earth-based  observations,  and  Veneras  4  and  5 
suggested  a  concentration  of  97%  carbon 
dioxide.  Radio-occultation  data  confirmed 
these  probe  measurements.  However, 
temperature  and  pressure  measure- 
ments from  probes  differed  from 
radio-occultation  measure- 
ments in  a  way  that  seemed 
best  explained  by 
supposing  that  Venus' 




Figure  1  -6.  The  first  picture  from 
the  surface  of  Venus,  obtained 
by  the  Soviet  spacecraft  Venera  9 
in  1 975,  shows  a  rocky  surface 
and  a  clear  view  to  the  horizon. 
The  rocks  appear  to  have  been 
fractured  and  broken  in  a 
geologically  recent  time. 

Bow  shock 




Figure  1-7.  Because  Venus 
does  not  have  a  magnetic 
field,  the  planet  interacts  much 
differently  with  the  solar  wind 
than  does  Earth.  This  simplified 
diagram  shows  the  expected 
configuration  as  scientists 
understood  it  before  the 
Pioneer  Venus  mission. 


70%  carbon  dioxide.  Also,  if  there  were  large 
amounts  of  argon  in  the  atmosphere,  carbon 
dioxide  could  be  as  low  as  25%  and  still  satisfy 
all  the  measurements  astronomers  made 
from  Earth. 

The  amount  of  carbon  dioxide  in  a  planetary 
atmosphere  affects  how  scientists  interpret  the 
planet's  microwave  spectrum.  With  accepted 
percentages  of  carbon  dioxide,  microwave 
observations  indicated  as  much  as  0.5%  water 
vapor  below  Venus'  clouds.  Instruments  on 
Veneras  9  and  10  provided  data  that  suggested 
0.1%  water  vapor  below  the  clouds.  At  the 
cloud  tops,  however,  they  indicated  only 
0.0001%  water  vapor.  Of  course,  there  was  the 
chance  that  if  the  atmosphere  contained 
another  gas  that  was  a  poor  absorber  of  micro- 
waves, the  planet's  atmosphere  could  contain 
even  more  water.  If  that  were  true,  scientists 
might  account  for  the  larger  amounts  of  water 
that  Veneras  4  and  5  measured  at  the  surface. 

On  the  other  hand,  the  spacecrafts'  measure- 
ments might  have  been  flawed — passage 
through  Venus'  sulfuric  acid  clouds  could  have 
contaminated  their  instruments. 

Carbon  dioxide  has  also  played  an  important 
role  in  the  evolution  of  the  planet's  atmo- 
sphere. And  it  affects  the  radiative  properties 
and  dynamic  traits  of  the  present  atmosphere. 
Despite  carbon  dioxide's  preponderance,  the 
total  amount  of  the  gas  seems  to  be  about  the 
same  as  that  locked  up  in  carbonate  rocks  in 
Earth's  crust. 

Upper  Atmosphere 

Observations  from  Earth  and  from  flyby  and 
orbiting  spacecraft  provided  data  on  the 
atmospheric  region  above  the  cloud  tops.  In 
contrast  with  the  lower  atmosphere,  this 
region  was  colder  and,  above  150  km  (93  miles), 
more  rarefied  than  Earth's  atmosphere. 

Because  Venus  lacks  a  significant  magnetic 
field,  the  solar  wind  interacts  directly  with  the 
upper  atmosphere  and  ionosphere  (Figure  1-7). 
Venus'  ionosphere  is  thinner  and  closer  to  the 
planet's  surface  than  is  Earth's.  Like  our  iono- 
sphere, Venus'  has  layers  where  the  electron 
density  peaks  (Figure  1-8).  Peak  electron 
density  in  Earth's  ionosphere  is  about  100,000 
to  1,000,000  electrons/cms  at  about  250  to 
300  km  (155  to  186  miles).  The  major  ion  is 
atomic  oxygen.  On  Venus,  by  contrast, 
scientists  measured  a  peak  of  about 
600,000  electrons/cms  at  about  142  km 
(88  miles).  The  major  ion  there  appeared  to 
be  molecular  oxygen. 

NASA's  Mariner  10  spacecraft,  which  in  1973 
flew  by  Venus  on  its  way  to  Mercury,  found 
two  clearly  defined  layers  in  the  nighttime 
ionosphere  (see  Figure  1-8):  a  main  layer  at 
142  km  (88  miles)  and  a  lesser  layer  at  124  km 
(77  miles).  The  lower  layer  had  a  peak  density 

about  75%  of  the  higher  layer.  Spacecraft  data 
revealed  a  sharp  boundary  (ionopause)  in  the 
dayside  ionosphere  at  350  km  (217  miles). 
Measurements  from  the  1967  Mariner  5 
spacecraft  had  placed  the  boundary  at  500  km 
(311  miles).  On  the  planet's  nightside,  the 
ionosphere  was  found  to  extend  high  into 
space,  probably  into  a  long  plasma  tail  stretch- 
ing away  from  the  Sun. 

Radio  occultation  data — measurements  of  a 
spacecraft's  radio  signal  as  it  disappears  behind 
the  planet — allowed  researchers  to  determine 
temperatures  in  the  region  just  above  the  cloud 
tops.  At  higher  altitudes,  in  the  exosphere, 
temperatures  were  determined  from  measure- 
ments of  radiated  ultraviolet  radiation  (air- 
glow).  Temperatures  at  the  top  of  the  Venusian 
ionosphere  required  a  gas  much  lighter  than 
carbon  dioxide.  Scientists  speculated  that  this 
gas  might  be  helium,  because  (1)  at  127°C 
(260°F)  or  so,  the  thermal  escape  of  helium 
from  the  atmosphere  would  be  small,  and  (2)  if 
helium  had  outgassed  from  Venus'  rocks  early 
in  its  history,  as  had  occurred  on  Earth,  then 
some  of  the  helium  would  likely  have  collected 
in  Venus'  upper  atmosphere.  Finally,  from 
both  infrared  and  ultraviolet  emission  mea- 
surements, researchers  discovered  a  corona  of 
hydrogen  atoms  beginning  at  about  800  km 
(497  miles)  altitude,  containing  up  to 
10,000  atoms/cm3. 


Mariner  10  photographed  at  least  two  layers  of 
extremely  wispy  haze  above  the  main  cloud 
deck — probably  layers  of  aerosols — 80  to  90  km 
(50  to  56  miles)  above  the  planet's  surface.  The 
layers  extended  from  equatorial  regions  to 
higher  latitudes. 

Scientists  did  not  understand  the  main  cloud 
layers'  composition.  In  fact,  the  clouds 
remained  controversial  until  the  early  1990s. 




1.0  X106 







-     ^^ 




2.5  X  TO* 


£    200 



1  6X104 



-  2.9X105  j     9X1Q3_ 



•       i       j     !      7X1Q3 


1.5  X104          1X104 



Day              Night 

Day               Night 



At  one  time,  astronomers  speculated  they  were 
dust  and  extended  down  to  the  surface. 
Another  speculation  was  that  they  were  con- 
densation clouds  with  a  clear  atmosphere 
beneath  them.  Suggested  components 
included  ammonium  nitrate,  carbon  suboxide, 
formaldehyde,  nitrogen  dioxide,  polymers  of 
hydrocarbonamide,  and  hydrochloric  acid. 

From  polarization  studies,  scientists  had 
concluded  by  1971  that  the  cloud  particles 
had  to  be  spherical,  about  1  to  2  microns  in 
diameter,  and  were  not  grains  of  dust.  Neither 

Figure  1  -8.  Made  before 
the  Pioneer  Venus  mission, 
these  graphs  compare  the 
ionosphere  layers  of  Earth 
and  Venus. 

Figure  1-9.  Scientists  inferred 
three  distinct  regions  of  Venus' 
atmosphere  from  several  sources. 
These  included  earlier  spacecraft 
flyby  missions,  Soviet  entry 
probes,  and  Earth-based 
observations.  You  can  see  these 
regions  in  the  diagram.  They  are 
the  high  atmosphere  above  the 
clouds,  the  thick  layer  of  clouds, 
and  the  clear  atmosphere 
beneath  the  clouds.  The  diagram 
also  shows  a  wind  velocity  profile 
to  illustrate  how  the  wind 
decreases  abruptly  at  the 
base  of  clouds. 







~    100 

Weakly  ionized  layer 




Main  ionization 

Weakly  ionized  layers 

v    Upper 

'  atmosphere 


Upper  hazes 
Lower  hazes 

Tropopause  clouds 

Wind  shear 

Low  hazes/aerosols/dust 

Clear  atmosphere  surface 




25  50  75 

Wind  speed  (m/sec) 


did  they  seem  to  be  ice  or  water  droplets,  nor 
droplets  of  hydrochloric  acid  or  carbon 

Scientists  now  accept  that  the  cloud  droplets 
are  composed  of  sulfuric  acid.  They  reached 
this  conclusion  in  1973  after  studying  mea- 
surements of  Venus'  infrared  spectrum  that 
had  been  made  with  instruments  aboard  a 
Learjet  high  in  Earth's  atmosphere.  Two 
theorists  had  suggested  this  composition 
earlier,  pointing  out  that  concentrated  sulfuric 
acid  is  a  very  effective  drying  agent  and  could 
account  for  the  atmosphere's  dryness  above 
the  cloud  tops. 

The  droplets  consist  of  about  a  75%  acid-water 
solution  and  are  about  1  micron  in  diameter. 
Sulfuric-acid  clouds  can  remain  as  clouds  over 
a  wider  range  of  temperatures  than  water 
clouds.  Below  the  bottom  of  main  cloud  layers, 
the  temperature  is  high  enough  for  sulfuric- 
acid  droplets  to  evaporate  into  water  and 
sulfuric-acid  vapors. 

While  Venus'  clouds  seem  opaque  from  Earth, 
they  are,  in  fact,  very  tenuous  but  deep  layers. 
Veneras  9  and  10  determined  that  visibility 
within  clouds  is  between  1  and  3  km  (0.6  to 
1.9  miles).  These  clouds  are  more  like  thin 
hazes  than  typical  clouds  on  Earth.  They  form 
a  very  deep  region  some  15  to  20  km  (9  to 

12  miles)  thick  (Figure  1-9).  This  is  more  than 
twice  the  thickness  of  cloud  layers  on  Earth. 
Venera  spacecraft  passed  through  several 
layers  and  emerged  from  the  cloud  deck's 
lower  boundary  at  about  49  km  (30  miles). 
Scientists  on  Earth  have  studied  distinctive, 
dark  ultraviolet  markings  on  the  clouds. 
These  are  probably  the  same  optical  markings 
that  early  observers  had  noted.  In  Figure  1-10 
you  can  see  horizontal  \|/-shaped  features. 
'1  hey  have  an  extension  of  the  equatorial  bar 
through  arms  that  are  sometimes  angular  and 
at  other  times  circular.  Features  that  look  like 
a  reversed  letter  C  appear  more  often  near  the 
evening  terminator  than  the  morning 
terminator.  Horizontal  Y-shaped  features 
sometimes  have  a  tail  stretching  round  the 
planet.  Sometimes  there  are  two  parallel 
equatorial  bands.  Patterns  are  mostly  sym- 
metrical about  the  equator.  Arms  of  the 
various  features  open  in  the  direction  of  their 
retrograde  motion,  which  varies  between  50 
and  130  m/sec  (164  and  427  ft/sec).  However, 
a  major  question  about  cloud  motions 
remained  unsolved:  did  they  result  from 
actual  movement  of  atmospheric  masses?  Or 
were  they  merely  a  wave  motion? 


Even  before  Pioneer's  in-depth  exploration 
of  Venus,  astronomers  had  determined  that 
Venus'  stratosphere  appears  to  have  a 

Figure  7-70.  Characteristic  cloud 
markings  on  Venus  appear  in 
three  drawings  at  the  right  and 
a  photograph  at  the  left. 
Astronomers  observed  the  C-,  Y-, 
and  \f/-shaped  markings  from 
Earth.  Mariner  1 0  images, 
returned  as  the  spacecraft  flew 
past  the  planet,  confirmed 
the  markings. 



continuous  zonal  motion  averaging  100  m/sec 
(328  ft/sec).  This  speed  indicates  a  rotation 
period  of  approximately  4  days,  which  is  60 
times  faster  than  the  planet's  own  spin.  This 
difference  in  speed,  relative  to  the  planet's 
surface,  causes  high-velocity  winds  to  blow 
continually  in  the  high  atmosphere.  Deeper  in 
the  atmosphere,  wind  velocities  decrease 
greatly,  dwindling  to  a  relative  calm  near  the 
surface.  The  Soviet  probes  showed  an  abrupt 
change  between  high-  and  low-wind  velocities 
at  about  56  km  (35  miles)  altitude.  This  change 
occurs  near  the  base  of  clouds.  Over  the  whole 
of  the  planet,  meridional  winds  of  much  less 
velocity  blow,  with  the  atmosphere  rising  at 
low  latitudes  and  sinking  toward  the  poles. 

Thermal  emission  from  the  upper  atmosphere 
differed  little  between  night  and  day  and 
between  low  and  high  latitudes.  This  showed 
there  is  strong  dynamic  activity  within  the 
atmosphere,  and  heat  in  large  amounts  is 
transferred  around  the  planet  horizontally  from 
day  to  night  and  from  equator  to  poles.  While 
diurnal,  or  daytime,  heating  is  important  above 
56  km  (35  miles),  dynamic  effects  prevail  below 
that  altitude. 

Magnetic  Field 

Venus'  lack  of  a  magnetic  field  is  another 
important  difference  between  it  and  Earth. 
Earth's  field  is  strong,  amounting  to  about 
0.5  gauss  at  its  surface.  In  1962,  Mariner  2,  the 
first  spacecraft  to  fly  by  Venus,  discovered  that 
Venus  has  no  significant  field.  In  fact,  Venus' 
field  strength  is  less  than  1/10,000  of  Earth's. 

Scientists  still  do  not  completely  understand 
how  planets  generate  and  maintain  their  mag- 
netic fields.  They  believe  a  self-sustaining 
dynamo  in  a  fluid  core  accounts  for  Earth's 
field.  Convection  currents  in  the  core  cause 
electric  currents,  and  these  produce  the  exter- 
nal magnetic  field.  This  theory,  which  seems  to 

apply  to  Jupiter,  Saturn,  Uranus,  and  Nep- 
tune, predicted  that  slow-spinning  satellites 
and  planets  without  molten  cores  do  not  have 
magnetic  fields.  However,  this  dynamo  theory 
failed  to  predict  slow-spinning  Mercury's 
magnetic  field,  discovered  by  Mariner  10. 

Lack  of  a  Satellite 

Several  astronomers  in  the  1800s  claimed 
discovery  of  a  "moon"  of  Venus.  However, 
their  satellites  turned  out  to  be  faint  stars. 
Venus  does  not  have  a  satellite. 

Early  Spacecraft  Missions  to  Venus 

Before  the  Pioneer  Venus  mission,  Venus  had 
been  the  target  for  13  spacecraft.  Three  of 
these  were  American  and  10  were  Russian.  Five 
were  flybys  and  8  were  landers.  Several  Russian 
missions  were  flybys  and  landers  that  sepa- 
rated before  reaching  Venus. 

Initial  Soviet  attempts  to  reach  Venus  with 
spacecraft  failed.  Then  came  the  spectacular 
190-day  voyage  of  NASA's  Mariner  2  in  1962. 
Mariner  2  was  America's  first  interplanetary 
spacecraft,  and  it  flew  within  34,833  km 
(21,645  miles)  of  the  planet. 

During  the  rest  of  the  1960s,  Russia  and 
America  used  two  different  methods  to  explore 
Venus.  The  Russians  flew  probe  and  lander 
missions  as  well  as  flybys.  The  United  States 
used  flybys  only.  The  two  countries  sometimes 
obtained  conflicting  information  about  Venus. 
For  example,  a  Soviet  Venera  4  lander  recorded 
a  surface  temperature  of  265°C  (510°F)  in  1967. 
In  the  same  year,  Mariner  5  flyby  experiments 
indicated  a  surface  temperature  of  527°C  (981°F). 
Atmospheric  pressure  calculations  did  not 
agree  either.  Later,  scientists  learned  why. 
Atmospheric  pressure  had  crushed  Venera  4  at 
an  altitude  of  about  34  km  (21  miles) — the 
probe  had  never  reached  the  surface. 

The  1969  Soviet  landers  were  structurally 
tougher,  but  even  they  failed  to  survive  the 
atmosphere's  intense  pressure.  The  Soviets 
finally  tasted  success  in  1970  when  Venera  7 
landed  on  Venus  and  returned  data  for 
23  minutes.  Later  in  the  1970s,  other  Soviet 
landers  returned  pictures  of  the  rock-strewn 
surface.  For  more  information  on  the  pre- 
Pioneer  Soviet  program,  turn  to  Chapter  7. 
Descriptions  of  major  findings  for  three 
American  flybys  appear  next. 

Mariner  2 

A  flyby  spacecraft,  Mariner  2  blasted  off  on 
August  27,  1962.  The  spacecraft  flew  within 
34,833  km  (21,645  miles)  of  Venus  on  Decem- 
ber 14,  1962.  Among  the  mission's  discoveries 
were  that  (1)  Venus  is  blanketed  by  cold,  dense 
clouds  about  25  km  (15.5  miles)  thick  with  a 
top  at  or  about  80  km  (50  miles);  (2)  the 
surface  temperature  is  at  least  425°C  (800°F)  on 
both  day  and  night  hemispheres;  and,  (3)  the 
planet  has  virtually  no  magnetic  field  or 
radiation  belts. 

Mariner  5 

A  flyby  spacecraft,  launched  June  14,  1967, 
Mariner  5  passed  Venus  at  3391  km  (2107  miles) 
on  October  19,  1967.  Occultation  experiments 
provided  readings  that  helped  scientists 
calculate  temperatures  of  527°C  (981°F)  and 
pressures  of  100  atmospheres  on  the  surface. 
Researchers  also  determined  detailed  iono- 
spheric structure  at  two  locations  on  the 
planet.  Using  an  ultraviolet  photometer,  they 
observed  very  low  exospheric  temperatures 
that  were  unexpected  and  difficult  to  explain. 

Mariner  10 

A  spacecraft  bound  for  Mercury,  Mariner  10 
passed  Venus  en  route.  NASA  launched  it  on 
November  3,  1973,  and  it  flew  past  Venus  at 
5793  km  (3600  miles)  on  February  5,  1974. 
Mariner  10  was  the  first  spacecraft  to  photo- 

graph Venus'  clouds.  Taken  in  ultraviolet 
light,  the  photographs  revealed  the  clouds' 
structural  details.  Mariner  10  also  confirmed 
the  reality  of  the  C-,  Y-,  and  x|/-shaped  mark- 
ings and  verified  the  4-day  rotation  period  of 
the  ultraviolet  markings.  The  spacecraft  found 
significant  amounts  of  helium  and  hydrogen 
in  the  upper  atmosphere.  Using  optical  limb 
scanning,  scientists  detected  high  altitude 
haze  layers  in  the  upper  atmosphere  above  the 
cloud  tops.  Mariner  10  confirmed  that  Venus 
lacks  a  magnetic  field  of  any  consequence, 
determined  the  structure  of  the  ionosphere, 
and  established  temperature  and  pressure 
profiles  into  the  upper  atmosphere. 

Unanswered  Questions 

Many  questions  about  Venus'  atmosphere 
remained  unresolved  at  the  time.  How  does 
the  Venus  weather  machine  work?  What 
makes  Venus  so  hot  compared  to  Earth — a 
greenhouse  effect?  Or  is  there  a  significant 
dynamic  contribution?  How  did  the  atmo- 
sphere of  Venus  evolve?  Did  Venus  once  have 
a  more  moderate  surface  temperature?  What 
caused  the  dark  ultraviolet  markings  in  Venus' 
clouds?  What  are  the  constituents  of  the 
atmosphere  at  different  levels? 

Scientists  believed  that  the  answers  to  such 
questions  would  help  us  learn  more  about  our 
own  Earth.  While  many  factors  complicate 
Earth's  meteorology  (mixing  of  oceanic  and 
continental  air  masses,  partial  cloud  cover, 
axial  tilt,  and  rapid  planetary  rotation),  Venus' 
meteorology  appeared  to  be  much  simpler. 
The  atmosphere  has  a  basic  composition  of 
97%  carbon  dioxide,  with  hardly  any  water. 
There  are  no  oceans  to  complicate  matters, 
and  because  the  planet  has  a  slow  rotation, 
Coriolis  forces  are  minor.  Since  its  spin  axis 
tilts  only  slightly,  there  are  virtually  no 
seasonal  effects. 



At  the  time  of  the  Venera  landings  in  1975, 
Louis  D.  Friedman  and  John  L.  Lewis  made 
several  important  observations.  They  pointed 
out  that,  despite  all  the  missions  to  Venus, 
some  of  the  most  important  and  fundamental 
scientific  questions  remained  unanswered.  For 
example,  very  few  early  results  helped  explain 
why  Venus  differs  so  much  from  Earth. 

Without  answers  to  basic  questions,  how  could 
we  learn  more  about  planetary  processes  and 
evolution?  We  needed  to  know  more  about 
Venus'  global  chemical  composition,  and  its 
thermal  and  differentiation  history.  This 
required  information  about  crustal  composi- 
tion, the  planet's  internal  structure,  and  the 
ages  of  crustal  rocks.  We  needed  to  know  if 
there  was  evidence  of  tectonic  activity,  conti- 
nental drift,  and  volcanism.  Mapping  of  the 
gravitational  field  in  local  regions  and  other 
geodetic  data  also  were  important,  as  was  the 
mapping  of  surface  features  to  determine  local 
geologic  structure.  We  needed  to  know  more 
about  atmospheric  composition,  thermal 
structure,  cloud  structure,  and  atmospheric 
circulation.  In  short,  early  spacecraft  observa- 
tions had  provided  intriguing  glimpses  in 
some  areas,  but  had  not  provided  much  relia- 
ble and  quantitative  information.  By  the  early 
1970s,  the  United  States  had  two  decades  of 
developing  reentry  vehicles  for  intercontinen- 
tal ballistic  missiles  (ICBMs).  The  space  program 
was  just  beginning  to  use  this  technology.  For 
example,  ICBM  research  provided  technology 
that  would  help  spacecraft  survive  high 
temperatures  and  deceleration  forces  in  Venus' 
atmosphere.  This  was  a  very  important  break- 
through. It  allowed  us  to  send  highly  sophisti- 
cated instruments,  already  demonstrated  on 
other  American  space  missions,  through 
Venus'  atmosphere  to  its  surface.  With  this 
technology,  scientists  now  could  take  a  new 
approach  to  exploring  the  cloud-shrouded 
planet.  The  time  was  perfect  for  Pioneer  Venus. 

On  March  15,  1973,  Richard  Goody  of  Harvard 
University  appeared  before  the  House  Com- 
mittee on  Science  and  Astronautics  to  discuss 
the  NASA  budget  authorization  for  fiscal  year 
1974.  During  his  talk,  he  repeated  a  statement 
he  had  made  before  the  Royal  Society  in 
London  on  the  500th  anniversary  of  the  birth 
of  Copernicus:  ". .  .  it  is  no  longer  possible  to 
consider  Earth  entirely  aside  from  the  other 
planets — planetary  science  has  grown  to  con- 
tain many  aspects  of  the  earth  sciences  and 
for  some  geophysicists  the  aim  of  inquiry 
has  now  become  the  nature  of  the  entire 
inner  Solar  System."  He  stressed  that 
observations  of  planets  such  as  Mars 
and  Venus  could  assist  some 
current  attempts  to  model  and 
predict  climatic  changes 
on  Earth. 

Although  no  one 

expected  Pioneer 

Venus  to  answer 

all  the  important 

quesions  about 

Venus,  it  has 

taken  us 

closer  to 

understanding  the 

planet  and  why  it  differs 

from  Earth.  Perhaps  the  most 

important  aspect  of  planetary 

exploration  is  to  learn  about  extreme 

cases  of  conditions  that  resemble  those  on 

Earth.  Venus  and  Mars  provide  these  needed 

comparisons  with  Earth.  NASA's  Pioneer  Venus 

program  and  the  Russian  Venera  program 

(before,  during,  and  after  Pioneer  Venus),  with 

data  from  NASA's  Magellan  program  in  the 

early  1990s,  have  provided  much  of  the 

information  needed  to  make  these  important 


Artist's  conception 
of  the  surface  of 




In  March  1959,  Warren  H.  Straly  of  the  Army 
Ballistic  Missile  Agency  presented  a  paper  at 
the  Hawthorne,  California,  meeting  of  the 
Lunar  and  Planetary  Exploration  Colloquium. 
The  meeting  took  place  at  the  Northrop  Cor- 
poration. Earlier,  considerable  emphasis  had 
been  on  the  planet  Mars  as  a  target  for  inter- 
planetary spacecraft.  Straly  compared  Mars 
missions  with  missions  to  Venus.  He  concluded 
the  latter  were  preferable  in  terms  of  overall 
energy  requirements  for  the  mission  and  for 
transmitting  data  back  to  Earth.  He  pointed 
out  that  astronomers  had  neglected  Venus, 
basically  because  Mars  was  a  more  interesting 
planet  to  observe.  With  telescopes, 

astronomers  could  see  the  surface 
of  Mars  and  observe  interesting 
changes  on  its  surface.  A  planet-wide 
cloud  system,  however,  hid  Venus' 
surface.  A  short  while  before  this 
meeting,  the  December  8,  1958,  issue  of 
Missiles  and  Rockets  magazine  had  a  related 
article.  It  reported  on  a  NASA  plan  to 
launch  a  spacecraft  to  Venus  in  June  1959. 
The  article  claimed  the  spacecraft  would  carry 
a  spectrometer,  a  magnetometer,  a  microwave 
detector,  and  other  instruments.  The  report 
said  that  the  launch  vehicle  was  to  be  a 
converted  ICBM  booster.  Unfortunately,  this 
mission  never  took  place.  However,  a  NASA 
spacecraft,  Mariner  2,  did  fly  by  Venus  in  1962. 
Another  flyby,  Mariner  5,  followed  it  in  1967. 
The  Soviets  tried  unsuccessfully  to  reach  Venus 
with  Sputnik  7  and  Venera  1  in  1961  and  with 
a  number  of  different  spacecraft  in  1962 
through  1964. 

The  Pioneer  Venus  project  began  shortly  after 
NASA's  Mariner  5  flew  by  Venus  and  Russia's 
first  successful  Venus  mission,  Venera  4,  probed 
the  planet's  atmosphere.  These  events  occurred 
in  October  1967.  Three  scientists— R.  M.Goody 

(Harvard  University),  D.  M.  Hunten  (Kitt  Peak 
National  Observatory),  and  N.  W.  Spencer 
(NASA  Goddard  Space  Flight  Center) — formed 
a  group  to  consider  the  possibility  of  a  simple 
entry  probe  to  investigate  Venus'  atmosphere. 
Goddard  Space  Flight  Center  awarded  a  study 
contract  to  AVCO  Corporation.  In  1968,  the 
Center  also  began  studying  capabilities  of 
small  planetary  orbiters  using  the  Explorer 
Interplanetary  Monitoring  Platform  (IMP) 
spacecraft.  (Thor-Delta  launch  vehicles  would 
carry  these  craft  into  space.)  Scientists  called 
the  proposed  mission  the  Planetary  Explorer. 

In  the  years  1967-1970,  scientists  had  few 
scientific  facts  on  which  to  base  plans  for  a 
Venus  mission.  Ground-based  observations 
had  added  very  little  to  their  knowledge  of 
the  planet.  In  addition,  the  few  spacecraft 
that  had  flown  near  Venus  had  returned  little 
new  information. 

Space  officials  admitted  their  methods  for 
exploring  Mars  and  our  own  Moon  would  be 
inadequate  for  Venus.  Before  Pioneer  Venus, 
scientists  designed  spacecraft  missions  mainly 
within  the  limits  of  existing  technology. 
Beginning  with  the  Venus  mission,  they 
adopted  a  new  view  that  looked  beyond  avail- 
able technology  for  future  missions.  Research- 
ers now  asked  key  scientific  questions  about 
Venus  and  then  defined  missions  and  new 
technologies  to  give  them  answers.  Using  this 
new  approach,  Venus-mission  scientists 
realized  that  spacecraft  payloads  should  not 
consist  of  individual  and  often  unrelated 
experiments  (as  they  had  in  past  missions). 
Instead,  experiments  would  apply  to  a  broad 
range  of  mission  goals. 

This  chapter  presents  a 
behind-the-scenes  look  at 
Pioneer  Venus'  early  days.  It 
covers  the  years  1967  to  the 
launch  of  Pioneers  12  and 
13  in  1978.  The  text 
discusses  the  program's  early 
studies  and  concerns.  For 
example,  what  is  the  most 
complete  scientific  payload? 
Who  will  design  the  launch 
vehicle  and  the  interplan- 
etary spacecraft?  How  would 
the  project  be  funded?  When 
will  be  the  best  launch 
dates?  As  you  read  about 
these  issues  and  watch  them 
evolve  into  Pioneer  Venus, 
you  also  meet  the  program's 
major  players. 



Early  Studies 

By  June  1965,  researchers  had  completed  a 
significant  study  (Planetary  Exploration  1969- 
1975)  with  backing  from  the  National  Acad- 
emy of  Sciences'  Space  Science  Board.  Their 
study  concluded  that  planetary  exploration 
should  be  wide-reaching.  Rather  than  a  space 
program  to  achieve  single,  isolated  goals,  the 
study's  authors  envisioned  one  that  covered  a 
broad  range  of  interrelated  scientific  disci- 
plines. Among  recommended  projects  were 
explorations  of  Venus  with  low-cost  spacecraft. 
Toward  this  goal,  the  Space  Science  Board 
recommended  that  NASA  start  a  program  of 
Pioneer/IMP-class  spinning  spacecraft  to  orbit 
Mars  and  Venus.  The  Board  also  suggested 
NASA  should  plan  missions  to  other  planets. 

Also,  during  the  summer  of  1965,  the  Space 
Science  Board  mounted  a  summer  study.  They 
later  issued  a  thick  report  entitled  Space 
Research:  Directions  for  the  Future.  R.  M.  Goody 
and  J.  Chamberlain  were  members  of  the 
Working  Group  on  Planetary  and  Lunar 
Exploration,  which  G.  MacDonald  chaired. 
The  panel  on  Venus  consisted  of  R.  M.  Goody, 
V.  Suomi,  and  G.  Wasserburg.  Their  recom- 
mendations for  space  probes  came  under  the 
headings  geodetic  measurements,  surface 
profile  (by  radio  altimetry),  cloud  structure, 
upper  atmosphere,  and  dropsondes.  The  panel 
recommended  specific  dropsonde  measure- 
ments. These  were  composition,  especially 
water  vapor  (with  a  suggestion  for  a  simple 
mass  spectrometer),  nature  of  clouds,  and 
intensity  of  solar  radiation  and  reradiated 
infrared  radiation  (to  test  the  greenhouse 
theory).  Their  suggestions  for  dropsondes  also 
included  some  sort  of  penetrometer,  to 
distinguish  between  solid  and  liquid  surfaces, 
and  a  seismometer.  So,  a  quarter  century  ago, 
researchers  had  already  earmarked  nearly  all 
the  instruments  for  the  Venus  probe  mission. 

Instruments  for  an  orbiter  also  were  clearly 
highlighted  in  these  early  studies. 

Unfortunately,  NASA  did  not  enthusiastically 
receive  these  ideas.  To  move  the  project 
forward,  R.  M.  Goody  started  a  campaign.  In 
1966,  he  sent  to  D.  M.  Hunten  a  paper  that 
was  an  exploratory  proposal  for  a  Venus  drop- 
sonde. By  early  1967,  Goody  had  enlisted 
D.  M.  Hunten  and  N.  W.  Spencer  into  an 
informal  consortium  to  help  define  the 
mission  and  the  instruments.  Spencer  was  a 
pioneer  in  exploration  of  Earth's  upper 
atmosphere.  His  specialties  included  sounding 
rockets  and  the  Explorer  series  of  satellites  that 
sampled  the  top  of  the  atmosphere.  Hunten 
was  a  specialist  in  instruments  and  was  well- 
versed  in  current  knowledge  about  Venus. 
Under  Goody's  leadership,  these  three  scien- 
tists recruited  other  experts  into  an  energetic 
group  that  pushed  strongly  for  an  advanced 
mission  to  Venus.  They  envisioned  a  mission 
that  would  orbit  the  planet  and  send  probes 
down  to  its  surface,  gathering  data  about  the 
atmosphere  as  they  descended. 

Goddard  Space  Flight  Center  published  its 
results  in  January  1969.  The  Center  recom- 
mended the  Venus  project  should  begin 
during  1973.  R.  M.  Goody,  D.  M.  Hunten, 
V.  Suomi,  and  N.  W.  Spencer  wrote  the  plan, 
A  Venus  Multiple-Entry  Probe  Direct-Impact 
Mission.  A  consortium  of  Harvard  University, 
Kitt  Peak  National  Observatory,  University  of 
Wisconsin,  and  Goddard  Space  Flight  Center 
proposed  the  study.  Besides  the  authors,  some 
25  scientists  added  to  the  study.  Goody 
pushed  this  report,  sending  copies  to  influen- 
tial science  writers.  He  appended  a  note  that 
"despite  its  Goddard  cover  it  is  a  piece  of 
private  enterprise  done  with  the  intention  of 
pushing  NASA  into  a  rational  planetary 
program  based  first  and  foremost  on  science 

objectives.  We  wanted  to  demonstrate  that  the 
objectives  on  Venus  could  be  rationally 
thought  out,  and  that  they  point  to  a  feasible 
mission,  which  I  hope  the  U.S.  may  adopt." 

Scientists  considered  several  different 
approaches  for  a  mission  to  Venus.  These 

approaches  included  a  buoyant  Venus  station 
(a  balloon  that  would  float  in  the  planet's 
atmosphere),  probes,  and  orbiters  (Figure  2-1). 
Mission  researchers  evaluated  pros  and  cons  of 
three  different  scenarios:  (1)  a  flyby  mission 
with  probe  release,  (2)  a  direct-impact  bus  with 
separate  probes  reaching  Venus  before  the  bus, 

6250  r- 











TV  and  thermal 

emission  microwave 

imager  operate 

from  bus 

Upper  atmosphere 


from  bus 


with  bus  ceases, 
large  probe  and 
balloon  probes 


data  stored  on 

large  probes  for 

relay  to  Earth 

Balloon  probes 
transmit  to  Earth 

Venus  4 


to  Earth 

Small  probes  and 

large  probe 
transmit  to  Earth 

Ro  radar  surface 


100  200  300  400  500  600 

Temperature  of  atmosphere  (K) 




Figure  2-1.  Regions  of  Venus' 
atmosphere  that  various  probes 
could  investigate  (from  the  first 
plan  for  a  comprehensive  mission 
to  the  cloud-shrouded  planet). 


Figure  2-2.  The  early  study 
compared  several  mission 
alternatives,  such  as  a  flyby 
and  a  direct  impact  mission 
for  release  of  probes.  The 
study  concluded  that  the 
latter  was  more  effective. 

Cruise  and 
release  probes 




Cruise  and 
release  probes 

Scan  planet. 

Make  upper 






'  \ 



and  (3)  an  orbiter  that  would  release  probes. 
After  careful  study,  scientists  concluded  that 
the  direct-impact  bus  mission  had  better 
chances  of  collecting  scientific  data  than  a 
flyby  mission  (Figure  2-2).  A  system  relying  on 
release  of  probes  from  a  planetary  orbiter  had 
its  advantages,  too — it  generated  lower 
temperatures  for  probes  entering  the  atmo- 
sphere. However,  a  planetary  orbiter  proved  to 
be  very  expensive.  To  launch  probes  and  an 
orbiter  around  Venus,  such  a  system  required 
too  much  propellant,  which  added  to  the 
spacecraft's  weight.  Complexity  and  cost  also 
ruled  out  large,  buoyant  stations  as  an  alterna- 
tive (at  least  until  more  details  of  the  Venusian 
atmosphere  became  available). 

Venus'  cloudy  atmosphere  was  an  effective 
barrier  to  its  surface  features.  The  Goddard 
report  suggested  that  a  system  of  three  small 
and  four  large  probes  could  solve  crucial 
problems  concerning  the  cloudy  atmosphere. 

These  included  the  nature  of  clouds  and  the 
structure,  chemistry,  and  motions  of  the 
atmosphere.  Ten  days  before  encounter,  three 
small  probes  could  enter  the  planet's  atmo- 
sphere near  the  subsolar  point,  the  antisolar 
point,  and  the  south  pole.  During  a  slow 
descent  to  the  surface,  the  three  probes  could 
take  specific  measurements.  These  measure- 
ments would  include  atmospheric  pressure, 
temperature,  and  a  component  of  the  horizon- 
tal wind.  Ninety  minutes  before  encounter, 
and  at  a  distance  of  five  Venus  radii— about 
30,000  km  (18,642  miles)— from  the  surface, 
bus  science  measurements  could  begin.  Probes 
could  take  television  and  microwave  thermal 
emission  pictures  of  the  planet  down  to  an 
altitude  of  135  km  (84  miles).  They  also  could 
measure  atmospheric  density,  electron  density, 
temperature,  day  airglow,  and  ion  and  neutral 
particle  composition. 


The  four  large  probes  could  leave  the  bus  at 
an  altitude  of  about  135  km  (84  miles).  This 
separation  would  happen  just  before  its  high- 
speed entry  into  the  atmosphere  destroyed  the 
bus.  Two  large  probes  could  be  identical  small 
balloons  that  would  carry  radar  transponders. 
The  balloons  could  float  in  the  atmosphere 
where  pressure  is  about  50  millibar,  or  about 
70  km  (43  miles)  above  Venus'  surface.  The 
radar  transponders  would  make  it  possible  to 
track  the  balloons  from  Earth.  While  scientists 
tracked  them,  the  balloons  would  measure 
pressure,  temperature,  solar  radiation  flux,  and 
upward  thermal  radiation  flux.  The  other  two 
large  probes  could  penetrate  toward  the 
surface.  They  would  measure  pressure,  tem- 
perature, gas  composition,  radiation  fluxes, 
cloud  particle  composition,  number  density, 
and  particle  size.  Perhaps  they  could  even 
reveal  physical  features  of  the  planet's  surface. 

The  Goddard  report  stated  probes  were  the 
only  way  to  take  measurements  crucial  for 
understanding  Venus'  atmosphere.  For  a 
given  cost,  the  report  concluded  that  the 
direct-impact  probe  could  achieve  a  real 
advantage  over  orbiting  and  flyby  spacecraft 
delivery  systems.  This  advantage  would 
translate  into  more  complete  atmospheric 
measurements  and  greater  reliability  in 
achieving  science  goals. 

In  1969,  Goddard  awarded  a  follow-on 
contract  to  AVCO  Corporation  to  study  a 
probe  mission  to  Venus  using  a  Thor-Delta 
launch  vehicle.  By  the  end  of  that  year,  NASA 
had  merged  the  concepts  into  a  universal  bus 
(a  combination  of  the  Venus  probe  spacecraft 
and  the  Planetary  Explorer  Orbiter  spacecraft). 
Their  idea  was  to  develop  a  spacecraft  that 
could  either  deliver  multiple  entry  probes  into 
the  Venusian  atmosphere  or  send  a  vehicle 
into  orbit  around  the  planet. 

The  "Purple  Book" 

In  1970,  21  scientists  of  the  Space  Science 
Board  and  the  Lunar  and  Planetary  Missions 
Board  of  NASA  studied  the  scientific  potential 
of  missions  to  Venus  based  on  the  technology 
amassed  from  experience  with  Explorer 
spacecraft.  They  produced  a  final  report, 
Venus — Strategy  for  Exploration,  which  became 
known  as  the  "Purple  Book"  because  of  its 
purple  cover. 

The  report  recommended  that  exploration  of 
Venus  should  be  a  NASA  goal  for  the  1970s 
and  1980s.  It  also  proposed  the  Delta- 
launched,  spin-stabilized  Planetary  Explorer 
spacecraft  as  the  main  vehicle  for  initial 
missions.  These  missions  would  include 
orbiters,  atmospheric  probes,  and  landers.  The 
report  stated  NASA  could  reduce  the  cost  of 
these  missions  if  the  agency  accepted  some 
higher  risks  than  in  previous  space  missions. 

The  report  outlined  a  strategy  to  explore 
Venus.  No  more  than  two  missions  would  be 
tried  at  each  launch  opportunity  when  the 
relative  positions  of  the  planets  made  a 
mission  possible  on  the  basis  of  an  available 
launch  vehicle  and  the  weight  of  the  science 
payload.  They  also  would  avoid  hybrid 
missions  because  of  their  complexity  and  cost. 
(A  hybrid  mission  might  be  a  spacecraft 
carrying  both  an  orbiter  and  an  atmospheric 
probe  or  a  lander.)  Missions  would  use  identi- 
cal payloads  wherever  possible. 

The  report  recommended  that  project  scien- 
tists carefully  weigh  the  scientific  value  of 
results  against  mission  costs.  The  strategy  was 
to  keep  mission  costs  at  a  minimum  (that  is, 
under  $200  million).  This  would  allow  NASA 
to  plan  a  series  of  missions  to  Venus.  The 
report  suggested  two  multiprobe  missions  for 
the  1975  opportunity  and  two  orbiters  for  the 



1976/77  opportunity.  Later  opportunities  were 
less  clear;  orbiters,  landers,  and  balloons  were 
all  candidates.  The  report  proposed  that  the 
1978  opportunity  should  be  a  follow-on 
landing  mission. 

This  1970  study  also  pointed  out  the  seeming 
paradox  of  differences  and  similarities  in  the 
evolution  of  the  two  planets.  It  claimed  that 
exploring  the  second  planet  from  the  Sun 
promised  to  reveal  new  insights  into 
planetary  evolution. 

Because  of  its  opaque  atmosphere  and  absence 
of  satellites,  scientists  knew  less  about  Venus 
in  1970  than  they  did  about  Mars.  Ideally, 
they  needed  measurements  of  Venus  to  deter- 
mine the  chemical  composition  and  mineral- 
ogy of  the  surface  materials,  the  heat  flux  from 
the  interior,  the  presence  or  absence  of  an 
iron-rich  core,  and  the  variation  of  elastic- 
wave  velocity  with  depth  and  with  wave  inten- 
sity. Making  such  measurements  on  Venus 
would  be  extremely  difficult  because  of  the 
high  temperature  at  the  planet's  surface — 
about  475°C  (887°F).  Nevertheless,  a  program 
of  measurements  on  a  scale  proposed  for 
Planetary  Explorers  would  allow  highly 
significant  measurements  to  be  made.  Surface 
elevations  could  be  measured  with  a  radar 
altimeter  on  an  orbiter,  and  some  information 
about  the  distribution  of  mass  in  the  planet 
could  be  obtained  from  the  way  in  which  the 
orbit  of  such  an  artificial  satellite  is  perturbed. 

The  "Purple  Book"  made  several  other  recom- 
mendations. It  suggested  that  NASA  continue 
to  support  and  develop  specific  Earth-based 
studies  of  Venus  to  complement  its  spacecraft- 
based  studies.  Among  these  techniques  were 
thermal  mapping  of  the  planet's  surface  by 
analysis  of  radio  emissions  from  the  surface, 
radar  topographical  mapping,  and  analysis  of 
radiation  from  cloud  tops.  The  report 

suggested  that  NASA  set  up  and  maintain  a 
continuous  group  to  (1)  plan  Venus  explora- 
tions, (2)  advise  on  strategy  for  these  missions, 
and  (3)  recommend  payloads  for  each  mission. 
The  study  stressed  the  need  for  a  wide  range 
of  novel  scientific  experiments  for  the  mis- 
sions, such  as  those  for  investigating  Venus' 
clouds.  In  a  summary  statement,  the  authors 
wrote,  "We  believe  that  the  combination  of 
scientific  goals  and  the  feasibility  of  con- 
tributing to  these  goals  makes  the  explora- 
tion of  Venus  one  of  the  most  important 
objectives  for  planetary  exploration  of  the 
1970s  and  1980s." 

Effect  of  the  Soviet  Venus  Probe, 
Venera  7 

In  the  fall  of  1970,  funding  a  new  program  for 
planetary  exploration  that  could  meet  a  1975 
launch  date  was  unlikely.  So,  planners  resched- 
uled the  entire  Venus  exploration  program. 
They  revised  the  plan  to  launch  two  multi- 
probe  spacecraft  during  the  1976/77  opportu- 
nity. In  addition,  they  planned  for  a  single 
orbiter  spacecraft  in  1978  and  a  single  multi- 
probe  (a  floating  balloon  probe  and  a  lander) 
in  1980. 

Soviet  scientists  also  were  extremely  interested 
in  exploring  Venus.  During  most  launch 
opportunities,  they  sent  spacecraft  to  the 
cloud-shrouded  planet  (see  Chapter  7).  They 
experienced  many  technical  difficulties,  and 
several  early  spacecraft  failed.  However,  their 
efforts  to  study  another  planet's  atmosphere 
were  partially  successful.  The  worldwide 
scientific  interest  they  created  more  than 
offset  their  failures. 

On  December  15,  1971— soon  after  the  Space 
Science  Board  published  its  1970  report — a 
Soviet  spacecraft,  Venera  7,  successfully 
entered  Venus'  atmosphere.  For  23  minutes,  it 
sent  data  from  the  surface.  In  view  of  these 

new  data,  scientists  asked  whether  the  recom- 
mendations of  the  1970  study  still  stood.  A 
special  panel  of  experts  met  to  reassess  the 
recommendations.  The  panel's  conclusion: 
The  Planetary  Explorer  program  recom- 
mended in  the  Venus  study  would  be  a 
well-articulated,  intensive  study  of  the 
planet  designed  to  attempt  to  answer  a 
list  of  first-order  questions.  Among  these 
are  the  number,  thickness,  and  composi- 
tion of  cloud  layers;  the  nature  of  the  cir- 
culation; explanation  of  the  high  surface 
temperature;  the  reason  for  the  lack  of 
water  and  the  remarkable  stability  of  the 
carbon  dioxide  atmosphere;  the  nature  of 
the  interaction  of  the  solar  wind  with  the 
planet;  the  elemental  composition  of  the 
surface;  the  distribution  of  mass  and  mag- 
netic field  strength;  and  the  measurement 
of  seismic  activity.  Venera  7  was  a  highly 
specialized  probe  designed  to  perform 
only  two  functions — to  measure  atmo- 
spheric temperature  and  pressure  down  to 
the  surface  of  Venus.  It  succeeded  in  mea- 
suring the  temperature  and  confirmed 
the  most  widely  held  expectation;  that 
the  surface  temperature  is  high.  It  has  in 
no  way  changed  the  conditions  on  which 
the  Venus  study  was  based  or  answered 
any  of  the  questions  that  planetary 
explorers  are  designed  to  answer.  We  can 
find  no  reason,  therefore,  to  recommend 
changes  in  the  scientific  objectives  set 
forth  in  previous  Board  studies. . . . 

Transfer  of  NASA's  Venus  Mission  to 
Ames  Research  Center 

Meanwhile,  NASA  had  continued  practical 
work  on  high-velocity  entry  of  spacecraft  into 
planetary  atmospheres.  By  1970,  research 
scientists  at  NASA  Ames  Research  Center  had 
gathered  much  experimental  data  about  effects 
on  bodies  moving  at  high  speed  in  an 

atmosphere.  Their  technique  was  to  photo- 
graph and  analyze  various  entry  shapes  in 
hypervelocity  free  flight  tunnels  at  speeds  up 
to  50,000  km/hr  (31,070  mph).  These  speeds 
were  higher  than  the  speed  needed  to  enter 
Venus'  atmosphere. 

By  1971,  Ames  Research  Center  had  designed, 
fabricated,  and  tested  a  spacecraft  and  most  of 
its  instrument  systems.  Engineers  designed  the 
equipment  to  demonstrate  selected  planetary 
experiments  and  instrumentation  in  Earth's 
atmosphere.  The  Planetary  Atmosphere 
Experiments  Test  (PAET)  was  a  vital  step  for 
future  missions.  It  established  a  technical  base 
for  advanced  planetary  exploration  of  Mars, 
Venus,  and  eventually  the  outer  planets.  A 
Scout  solid-propellant  multistage  rocket 
launched  the  test  spacecraft.  The  launch 
vehicle's  third  and  fourth  stages  carried  the 
PAET  spacecraft  back  into  Earth's  atmosphere 
at  24,000  km/hr  (14,914  mph). 

Launched  at  3:31  p.m.  EOT  on  June  20,  1971, 
the  test  spacecraft  was  highly  successful.  Just 
as  experimenters  planned,  instruments 
scooped  up  atmospheric  gases.  Even  more 
important,  PAET  demonstrated  the  capability 
of  selected  experiments  to  determine  structure 
and  composition  of  an  unknown  planetary 
atmosphere  from  a  high-speed  entry  probe. 
This  was  the  type  of  practical  data  researchers 
needed  to  design  a  probe  that  could  enter 
Venus'  atmosphere.  The  PAET  program  proved 
the  capability  of  Ames  Research  Center 
personnel  to  participate  in  such  a  mission. 
Meanwhile,  in  July  1971,  NASA  issued  an 
Announcement  of  Opportunity  (AO)  for 
scientists  to  participate  in  defining  the  Venus 
program.  In  November  of  that  year,  NASA 
discontinued  the  Planetary  Explorer  program 
at  Goddard.  By  January  1972,  the  agency  had 
transferred  it  to  Ames  Research  Center,  Moffett 
Field,  California.  At  Ames,  a  study  team 


Pioneer  Venus  Study  Team 

R.  R.  Nunamaker  (chair), 
H.  F.  Matthews,  M.  Erickson, 
T.  N.  Canning,  D.  Chisel, 
R.  A.  Christiansen,  L.  Colin, 
].  Cowley,  J.  Givens, 
T.  Grant,  W.  L.  Jackson, 
T.  Kato,  J.  Magan, 
/.  Mulkem,  L.  Polaski, 
R.  Ramos,  S.  Sommer, 
J.  Sperans,  T.  Tenderland, 
N.  Vojvodich,  M.  Wilkins, 
L.  Yee,  E.  Zimmerman 


quickly  organized  itself  and  the  project  was 
renamed  Pioneer  Venus. 

This  team  defined  the  system  and  worked 
closely  with  a  Pioneer  Venus  Science  Steering 
Group  made  up  of  interested  scientists  to 
define  the  mission's  scientific  payloads. 

Science  Steering  Group  and  the 
"Orange  Book" 

NASA  established  this  Pioneer  Venus  Science 
Steering  Group  in  January  1972.  The  group's 
purpose  was  to  enlist  widespread  science 
community  participation  in  the  early  selection 
of  the  mission's  science  requirements.  The 
Science  Steering  Group  met  with  Pioneer 
Venus  project  personnel  from  February 
through  June  1972.  They  developed  in  detail 
the  scientific  rationale  and  objectives  for  the 
early  Venus  missions.  The  group  also  con- 
ceived and  planned  candidate  payloads  and 
spacecraft.  Their  efforts  provided  a  useful 
guide  for  the  NASA  Payload  Selection  Commit- 
tee and  for  the  contractors  who  would  later 
develop  the  payloads  and  spacecraft. 

During  the  first  five  months  of  its  operations, 
the  Science  Steering  Group  held  several  meet- 
ings. In  1972,  the  group  published  a  compre- 
hensive report  that  became  the  accepted  guide 
to  Venus  exploration.  Known  as  the  "Orange 
Book"  (again  because  of  the  cover's  color),  the 
report  carefully  reviewed  and  endorsed  the 
scientific  rationale  for  missions  to  Venus.  It 
based  its  reviews  and  endorsements  on 
developments  since  the  earlier  Space  Science 
Board's  1970  report,  Venus — Strategy  for 
Exploration.  These  developments  included 
delays  in  starting  the  program,  scientific  find- 
ings from  the  Soviet  probe  Venera  7,  new 
Earth-based  observations,  new  theoretical 
analyses,  and  continued  analysis  of  data  that 
earlier  Soviet  and  American  spacecraft  gath- 
ered. The  report  recommended  that  missions 

continue  with  multiple  probes  in  1976/77.  It 
also  suggested  a  single  orbiter  in  1978  followed 
by  a  probe-type  mission  in  1980. 

The  Science  Steering  Group's  report  stated  that 
most  scientific  questions  about  Venus  required 
in  situ  atmospheric  measurements.  Measure- 
ments should  start  at  the  cloud  tops  and 
extend  as  far  as  possible  toward  the  surface. 
The  group  defined  24  important  questions 
about  Venus  (Table  2-1). 

The  required  technology  and  scientific 
instruments  needed  for  the  mission  were  con- 
sidered state-of-the-art  at  that  time.  Therefore, 
researchers  believed  a  probe  mission  at  the  first 
opportunity  was  desirable.  In  case  of  a  failure, 
they  suggested  a  dual  launch  mission.  If  both 
spacecraft  were  successfully  launched,  they 
recommended  retargeting  the  second  probe 
based  on  what  they  learned  from  the  first.  To 
ensure  the  best  chance  for  success,  the  group 
suggested  a  third  probe  for  the  final  launch 

The  study  recommended  that  the  first  mission 
should  consist  of  two  identical  spacecraft  and 
payloads.  These  would  be  ready  for  launch 
from  December  1976  through  January  1977. 
Each  spacecraft  would  consist  of  a  bus,  a  large 
probe,  and  three  small  probes.  The  large 
probes  would  have  parachutes;  the  small 
probes  would  be  free-falling  and  identical.  The 
spacecraft  would  be  spin-stabilized  and  would 
use  solar  power.  Cruise  from  Earth  to  Venus 
would  take  about  125  days.  The  probes  would 
separate  from  the  bus  about  10  to  20  days 
before  entry  into  the  Venusian  atmosphere.  In 
addition  to  transporting  the  probes,  the  buses 
also  would  enter  the  Venusian  atmosphere  (at 
shallow  angles)  and  send  back  data  until  they 
burned  up.  Their  mission:  to  gather  informa- 
tion about  the  upper  atmosphere. 

Table  2-1.  Questions  by  Science  Steering  Croup  for  Pioneer  Venus  Mission 

1 .  Cloud  layers:  What  is  their  number,  and  where  are  they  located?  Do  they  vary  over 
the  planet? 

2.  Cloud  forms:  Are  they  layered,  turbulent,  or  merely  hazes? 

3.  Cloud  physics:  Are  the  clouds  opaque?  What  are  the  sizes  of  the  cloud  particles?  How 
many  particles  are  there  per  cubic  centimeter? 

4.  Cloud  composition:  What  is  the  chemical  composition  of  the  clouds?  Is  it  different  in 
the  different  layers? 

5.  Solar  heating:  Where  is  the  solar  radiation  deposited  within  the  atmosphere? 

6.  Deep  circulation:  What  is  the  nature  of  the  wind  in  the  lower  regions  of  the  atmosphere? 
Is  there  any  measurable  wind  close  to  the  surface? 

7.  Deep  driving  forces:  What  are  the  horizontal  differences  in  temperature  in  the  deep 

8.  Driving  force  for  the  4-day  circulation:  What  are  the  horizontal  temperature  differences  at 
the  top  layer  of  clouds  that  could  cause  the  high  winds  there? 

9.  Loss  of  water:   Has  water  been  lost  from  Venus?  If  so,  how? 

10.  Carbon  dioxide  stability:  Why  is  molecular  carbon  dioxide  stable  in  the  upper  atmosphere? 

1 1 .  Surface  composition:  What  is  the  composition  of  the  crustal  rocks  of  Venus? 

1 2.  Seismic  activity:  What  is  its  level? 

1  3.  Earth  tides:   Do  tidal  effects  from  Earth  exist  at  Venus,  and  if  so,  how  strong  are  they? 

1 4.  Gravitational  moments:  What  is  the  figure  of  the  planet?  What  are  the  higher  gravitational 

1 5.  Extent  of  the  4-day  circulation:   How  does  this  circulation  vary  with  latitude  on  Venus 
and  depth  in  the  atmosphere? 

1 6.  Vertical  temperature  structure:  Is  there  an  isothermal  region?  Are  there  other  departures 
from  adiabaticity?  What  is  the  structure  near  the  cloud  tops? 

1  7.  Ionospheric  motions:  Are  these  motions  sufficient  to  transport  ionization  from  the  day 
to  the  night  hemisphere? 

1 8.  Turbulence:  How  much  turbulence  is  there  in  the  deep  atmosphere  of  the  planet? 

19.  Ion  chemistry:  What  is  the  chemistry  of  the  ionosphere? 

20.  Exospheric  temperature:  What  is  the  temperature  and  does  it  vary  over  the  planet? 

21 .  Topography:  What  features  exist  on  the  surface  of  the  planet?  How  do  they  relate  to 
thermal  maps? 

22.  Magnetic  moment:  Does  the  planet  have  any  internal  magnetism? 

23.  Bulk  atmospheric  composition:  What  are  the  major  gases  in  the  Venus  atmosphere? 
How  do  they  vary  at  different  altitudes? 

24.  Anemopause:   How  does  the  solar  wind  interact  with  the  planet? 


The  1978  launch  would  be  an  orbiter  mission. 
Generating  electrical  power  from  solar  cells, 
the  spacecraft  would  be  spin-stabilized.  It 
would  be  launched  between  May  and  August 
1978.  After  its  interplanetary  cruise,  the 
spacecraft  would  go  into  an  elliptical  orbit 
around  Venus.  Engineers  would  design  the 

spacecraft  to  orbit  the  planet  for  a  Venus 
sidereal  day  (243.1  Earth  days).  Major  goals 
would  be  to  (1)  produce  a  global  map  of  the 
Venusian  atmosphere  and  ionosphere,  (2)  get 
measurements  directly  from  the  upper 
atmosphere  and  its  ionosphere,  (3)  investigate 
interactions  between  solar  wind  and 


ionosphere,  and  (4)  study  the  planet's  surface 
by  remote  sensing. 

The  Steering  Group  still  contemplated  a  third 
probe  mission  for  1980.  They  expected  details 
of  a  1980  mission  to  become  clearer  as  two 
things  happened:  (1)  as  they  more  clearly 
defined  the  1976/77  mission  and  (2)  as  they 
later  reviewed  that  mission's  results.  The 
study  made  no  recommendations  for  a 
launch  in  1982. 

Despite  Russian  entry  probes  and  flybys, 
scientists  knew  very  little  about  Venus'  lower 
atmosphere  in  1972.  For  example,  they  did  not 
know  how  many  cloud  layers  there  were,  how 
thick  they  were,  or  what  was  in  them.  There 
were  at  least  three  very  different  hypotheses  to 
explain  the  planet's  high  surface  temperature. 

After  an  independent  study  of  the  Soviet 
Venus  program,  the  Science  Steering  Group 
agreed  with  the  Space  Science  Board's  earlier 
assessment  of  the  Venera  program.  The 
previous  1 1  years  of  Soviet  exploration  of 
Venus  had  produced  direct  measurements  of 
the  lower  atmosphere.  These  measurements 
included  pressure,  temperature,  density,  and 
gross  atmospheric  composition.  The  National 
Academy  of  Sciences'  Venus  study,  however, 
exposed  a  wide  range  of  scientific  problems 
that  the  Soviet  programs  had  not  tackled. 
Among  them  were  questions  about  the 
magnetosphere,  upper  atmosphere,  lower 
atmosphere,  and  the  solid  planet. 

When  it  came  to  recommending  instruments 
for  the  spacecraft,  the  Science  Steering  Group 
adopted  a  conservative  approach  to  avoid 
increasing  costs.  The  group  decided  that 
acceptable  instruments  should  have  already 
performed  successfully  in  Earth's  atmosphere. 
They  also  agreed  experiments  should  not  use 
novel  concepts  of  measurement.  Wherever 

possible,  instruments  should  already  qualify 
for  spacecraft  or  aircraft  use.  If  they  did  not 
already  qualify,  instruments  had  to  be  simple 
and  rugged.  Only  if  they  performed  satisfacto- 
rily in  laboratory  tests  should  they  be  consid- 
ered for  the  Venus  missions. 

The  Pioneer  Venus  Mission 

The  Pioneer  Venus  program  began  as  a  model, 
low-cost  program.  It  developed  around 
innovative  approaches  to  management  and  an 
understanding  that  the  total  cost  would 
remain  below  $200  million.  The  program 
crystallized  as  a  single-opportunity  mission. 
Consisting  of  a  Multiprobe  spacecraft  and  an 
Orbiter  spacecraft,  it  reflected  significant, 
major  advances  in  the  sophistication  of 
spacecraft  and  instruments  compared  with 
earlier  Venus  spacecraft. 

The  Pioneer  Venus  Multiprobe  would  be  an 
important  step  in  answering  questions  about 
the  planet's  atmosphere.  It  would  provide  data 
about  the  cloud  layers,  their  forms,  physics, 
and  composition.  It  would  investigate  the 
atmosphere's  bulk  composition,  its  solar  heat- 
ing, deep  circulation  and  driving  forces,  its  loss 
of  water,  the  stability  of  carbon  dioxide,  and 
the  vertical  temperature  structure.  Also  included 
would  be  data  on  ionospheric  turbulence,  ion 
chemistry,  exospheric  temperature,  magnetic 
moment,  and  the  anemopause  where  the  solar 
wind  reacts  with  the  planet's  atmosphere. 

The  Pioneer  Venus  Orbiter  also  would  provide 
significant  information  about  cloud  forms, 
cause  of  the  four-day  circulation,  loss  of  water, 
gravitational  moments,  extent  of  the  four-day 
circulation,  vertical  temperature  structure, 
ionospheric  motions,  ion  chemistry 
exospheric  temperature,  topography,  magnetic 
moment,  bulk  atmospheric  composition,  and 
the  anemopause. 

European  Study 

Early  in  1972,  members  of  the  European  Space 
Research  Organization  (ESRO)  asked  to 
participate  in  the  1975  Orbiter  mission.  A 
meeting  took  place  in  April  1972,  which 
included  NASA  and  ESRO  members.  Attendees 
decided  to  examine  jointly  how  the  two 
organizations  could  work  together  on  the  1978 
Venus  Orbiter  mission.  NASA  would  produce 
and  provide  ESRO  with  the  Orbiter  version  of 
the  basic  spacecraft,  or  Bus,  together  with 
common  equipment.  ESRO  would  then  adapt 
the  Bus,  including  a  retromotor  to  slow  the 
spacecraft  as  it  approached  Venus.  (The  retro- 
motor  would  allow  it  to  enter  an  orbit  around 
the  planet.)  Also,  ESRO  would  provide  a  high- 
gain  antenna  to  allow  communications  at  high 
data  rates  and  would  integrate  scientific  experi- 
ments. In  addition,  the  European  group  would 
undertake  qualification  tests  on  the  spacecraft 
and  its  payload.  NASA  would  then  accept  the 
Orbiter  for  launch  and  flight  operations. 

To  define  the  objectives  for  a  Venus  Orbiter 
launch  in  1978,  a  Joint  Working  Group  of 
European  and  U.S.  scientists  formed.  The 
scientists  met  periodically  and  issued  a  report 
in  January  1973,  Pioneer  Venus  Orbiter.  This 
report  recalled  that  a  series  of  missions  had 
been  proposed  since  the  start  of  the  NASA 
Venus  exploration  concept.  A  series  combining 
orbiter  and  probe  capabilities  was  the  favored 
method  for  exploring  Venus'  environment.  By 
mid-1972,  the  group  had  defined  the  present 
mission  series.  They  called  for  a  Multiprobe 
mission  in  the  1976/77  launch  opportunity 
and  for  an  Orbiter  mission  in  1978.  The 
science  experiments  for  the  Orbiter  mission 
required  a  highly  inclined  orbit  plane — greater 
than  90°  with  respect  to  the  ecliptic,  the  plane 
of  Earth's  orbit.  According  to  the  Working 
Group,  a  low  periapsis  (the  point  in  its  orbit 
where  the  Orbiter  would  be  nearest  Venus)  of 
200  km  (125  miles)  or  less  was  desirable.  The 

periapsis  would  be  at  about  latitude  45°,  initially 
in  the  sunlit  hemisphere.  Solar  gravity  would 
cause  the  periapsis  altitude  to  increase.  To  keep 
the  altitude  in  a  desired  range  would  require 
periodic  orbital  change  maneuvers.  Apoapsis 
(the  point  in  its  orbit  where  the  Orbiter  would 
be  farthest  from  Venus)  would  be  at  60,000  to 
70,000  km  (37,284  to  43,498  miles).  Drag  at 
periapsis  would  decrease  the  apoapsis  altitude 
and  reduce  the  period  in  orbit,  which  would 
initially  be  close  to  24  hours.  Maneuvers 
would  be  needed  to  maintain  the  period. 

Researchers  also  defined  experiments  and 
specified  required  characteristics  of  scientific 
instruments.  They  described  three  science 
payloads,  depending  on  how  much  scientific 
payload  the  spacecraft  could  carry. 

The  Working  Group  stated  that,  in  general,  a 
model  payload  should  consist  of  instruments 
to  measure  four  important  areas  of  interest 
about  Venus: 

(1)  Interaction  of  the  solar  wind  with  the 
ionosphere  would  be  investigated  by  a  magne- 
tometer, a  solar  wind  and  photoelectron 
analyzer,  an  electric  field  detector,  and  an 
electron  and  ion  temperature  probe. 

(2)  Aeronomy  and  the  airglow  would  be 
investigated  by  a  neutral  mass  spectrometer, 
an  ion  mass  spectrometer,  and  an  ultraviolet 
spectrometer/photometer  (aeronomy  includes 
investigating  atmospheric  composition  and 

(3)  The  atmosphere's  thermal  structure  and 
lower  atmospheric  density  would  be  investi- 
gated by  an  infrared  radiometer  and  a  dual- 
frequency  (S-  and  X-band)  occultation 

(4)  Surface  topography,  reflectivity,  and 
roughness  would  be  investigated  with  a 
radar  altimeter. 



The  Group  considered  several  other  instru- 
ments and  experiments.  These  included  a 
microwave  radiometer  to  map  thermal 
emission  from  the  planet's  surface,  an  electric 
field  sensor  to  detect  plasma  waves  generated 
by  the  interaction  of  the  solar  wind  with  the 
ionosphere,  a  solar  ultraviolet  occultation 
experiment,  and  a  photopolarimeter. 

Scientists  were  extremely  interested  in  deter- 
mining Venus'  gravitational  field  and  geo- 
metrical shape.  Such  information  is  important 
to  our  understanding  of  the  origin  and 
evolution  of  the  Solar  System's  inner  planets. 
It  also  helps  us  determine  why  Earth  and 
Venus  evolved  so  differently.  Gravitational 
experiments  require  an  orbiter  with  a  periapsis 
high  enough  to  avoid  any  atmospheric  drag. 
They  also  require  one  capable  of  remaining  in 
orbit  long  enough  to  gather  many  data  points 
of  tracking.  Unfortunately,  there  was  a  conflict 
between  in  situ  measurements,  requiring  a  low 
periapsis,  and  gravitation  measurements, 
requiring  a  high  periapsis.  To  resolve  this  con- 
flict, the  Working  Group  recommended  that 
the  mission  go  beyond  the  nominal  243  days. 
The  extra  days  would  allow  experimenters  to 
make  accurate  gravity  measurements. 

Later,  the  Managing  Executive  Council  for 
ESRO  voted  not  to  participate.  But  they  made 
this  vote  only  after  the  European  Space 
Organization  had  made  valuable  contributions 
to  the  program's  development.  These  contribu- 
tions included  important  studies  at 
Messerschmitt-Bblkow-Blohm  and  at  the 
British  Aerospace  Company. 

Pioneer  Venus  Science  Payload 

Meanwhile,  during  the  ESRO  study,  NASA 
made  a  decision  in  August  1972  to  restrict  the 
program  to  two  flights  only.  The  flights  would 
be  a  Multiprobe  at  the  first  opportunity  (1977) 
and  an  Orbiter  at  the  second  opportunity 

(1978).  In  September  1972,  NASA  issued  an  AO 
for  scientists  to  participate  in  the  Multiprobe 
mission.  In  addition  to  investigators  who 
would  develop  hardware  for  the  scientific 
instruments,  NASA,  for  the  first  time,  invited 
interdisciplinary  scientists  and  theoreticians  to 
participate.  After  learning  about  the  AO,  the 
Science  Steering  Group  disbanded.  This  deci- 
sion freed  the  members  from  conflicts  of 
interest  so  they  could  respond  to  the  AO  if 
they  so  chose. 

Mission  scientists  selected  the  preliminary 
payload  for  the  Multiprobe  mission  in  April 
1973.  An  AO  for  the  Orbiter  mission  followed 
this  selection  in  August  1973.  During  the 
following  months,  a  NASA  Instrument  Review 
Committee  reviewed  instrument  design  studies 
for  the  Multiprobe  mission.  They  also  consid- 
ered proposals  for  the  Orbiter's  scientific 
payloads.  NASA  headquarters  received  recom- 
mendations in  May  1974  and  finalized  the 
payloads  on  June  4,  1974. 

Scientists  chose  12  instruments  for  the  Orbiter, 
7  for  a  Large  Probe,  3  identical  instruments  for 
each  of  four  Small  Probes,  and  2  for  the  Multi- 
probe  Bus.  In  addition,  they  chose  several 
radio-science  experiments  that  were  applicable 
to  all  spacecraft  (Table  2-2). 

During  the  early  program,  a  total  of  114 
scientists  were  involved.  Science  management, 
however,  was  restricted  to  a  smaller  group. 
This  group  consisted  of  the  principal  investiga- 
tors, a  radio-science  team  leader,  a  radar  team 
leader,  interdisciplinary  scientists,  and  pro- 
gram and  project  scientists.  These  individuals 
comprised  a  new  Science  Steering  Group  under 
the  chairmanship  of  T.  M.  Donahue  and 
co-chairmanship  of  D.  M.  Hunten,  L.  Colin, 
and  R.  F.  Fellows.  (On  his  retirement  in  1978, 
the  program  scientist,  R.  F.  Fellows,  was 
succeeded  by  R.  Murphy  and  then  H.  Brinton.) 

Table  2-2.   Science  Instruments:  Project  Acronyms  and  Principal  Investigators 

Composition  and  Structure  of  the  Atmosphere 

Large  Probe  Mass  Spectrometer  (LNMS), ).  Hoffman 
Large  Probe  Gas  Chromatograph  (LGC),  V.  Oyama 
Bus  Neutral  Mass  Spectrometer  (BNMS),  U.  Von  Zahn 
Orbiter  Neutral  Mass  Spectrometer  (ONMS),  H.  Niemann 
Orbiter  Ultraviolet  Spectrometer  (OUVS),  I.  Stewart 
Large/Small  Probe  Atmosphere  Structure  (LAS/SAS),  A.  Seiff 
Atmospheric  Propagation  Experiments  (OGPE),  T.  Croft 
Orbiter  Atmospheric  Drag  Experiment  (OAD),  G.  Keating 


Large/Small  Probe  Nephelometer  (LN/SN),  B.  Ragent 

Large  Probe  Cloud  Particle  Size  Spectrometer  (LCPS),  R.  Knollenberg 

Orbiter  Cloud  Photopolarimeter  (OCPP),  j.  Hansen  (later  L.  Travis) 

Thermal  Balance 

Large  Probe  Solar  Flux  Radiometer  (LSFR),  M.  Tomasko 
Large  Probe  Infrared  Radiometer  (LIR),  R.  Boese 
Small  Probe  Net  Flux  Radiometer  (SNFR),  V.  Suomi 
Orbiter  Infrared  Radiometer  (OIR),  F.  Taylor 


Differential  Long  Baseline  Interferometry  (DLBI),  C.  Counselman 

Doppler  Tracking  of  Probes  (MWIN),  A.  Kliore 

Atmospheric  Turbulence  Experiments  (MTUR/OTUR),  R.  Woo 

Solar  Wind  and  Ionosphere 

Bus  Ion  Mass  Spectrometer  (BIMS),  H.  Taylor 

Orbiter  Ion  Mass  Spectrometer  (OIMS),  H.  Taylor 

Orbiter  Electron  Temperature  Probe  (OETP),  L.  Brace 

Orbiter  Retarding  Potential  Analyzer  (ORPA),  W.  Knudsen 

Orbiter  Magnetometer  (OMAG),  C.  Russell 

Orbiter  Plasma  Analyzer  (OPA),  j.  Wolfe  (later  A.  Barnes) 

Orbiter  Electric  Field  Detector  (OEFD),  F.  Scarf 

Orbiter  Dual-Frequency  Occultation  Experiments  (ORO),  A.  Kliore 

Surface  and  Interior 

Orbiter  Radar  Mapper  (ORAD),  G.  Pettengill 

Orbiter  Internal  Density  Distribution  Experiments  (OIDD),  R.  Phillips 

Orbiter  Celestial  Mechanics  Experiments  (OCM),  I.  Shapiro 

High  Energy  Astronomy 

Orbiter  Gamma  Burst  Detector  (OGBD),  W.  Evans 


To  deal  with  specific  subjects,  various  commit- 
tees formed  among  the  scientists.  Several  of 
these  were  long  standing,  including  six 
Working  Groups  for  each  scientific  area  of 
investigation.  Before  launch,  they  developed 
key  questions,  and  afterward  they  synthesized 
the  results  they  received  from  the  spacecraft. 
Another  very  active  group  was  concerned  with 
mission  operations  planning  for  the  Orbiter. 

This  group,  called  the  OMOP  Committee  (for 
Orbiter  Mission  Operations  Planning  Commit- 
tee) consisted  of  H.  Masursky,  L.  Colin, 
T.  M.  Donahue,  R.  O.  Fimmel,  D.  M.  Hunten, 
G.  H.  Pettengill,  C.  J.  Russell,  N.  W.  Spencer, 
and  A.  I.  Stewart.  H.  Masursky  served  as 
chairman  of  OMOP  until  his  death,  at  which 
time  D.  Hunten  succeeded  him. 


Six  Working  Groups  developed  key  scientific 
questions.  Chairmanship  of  these  groups 
varied  during  the  mission,  but  the  major  lead- 
ers were  J.  Hoffman,  composition  and  struc- 
ture of  the  Venus  atmosphere;  R.  Knollenberg, 
clouds;  M.  Tomasko,  thermal  balance; 
G.  Schubert,  dynamics;  S.  Bauer,  solar  wind 
and  ionosphere;  and  H.  Masursky,  surface 
and  interior. 

The  mission  procured  instruments  in  several 
ways.  Usually,  the  principal  investigator  was 
responsible  for  having  a  particular  instrument 
built.  He  could  either  (1)  build  it  in  his  own 
laboratory,  (2)  subcontract  its  construction,  or 
(3)  use  a  combination  of  these  methods. 

According  to  the  second  scenario,  the 
Pioneer  project  office  would  contract  some 
industrial  firm  and  then  monitor  how  the 
firm  developed  the  instrument.  During  this 
process,  the  principal  investigator  still  partici- 
pated to  assure  that  it  met  the  requirements 
of  his  experiment. 

As  an  example  of  the  third  scenario,  the 
project  office  built  the  Orbiter's  radar  mapper 
for  a  radar  team.  Carl  Keller,  an  Ames  Research 
Center  engineer,  had  overall  decision-making 
responsibility.  Hughes  Aircraft  built  the  radar 
as  a  result  of  an  open  bid  procurement. 

There  was  much  talk  at  the  beginning  of  the 
program,  before  the  AO  went  out,  that  the 
mission  would  use  only  instruments  that  had 
flight-proven  capability.  The  instruments  had 
to  have  flown  in  other  spacecraft  or  in  Earth's 
atmosphere.  This  requirement  was  intended  to 
save  money  and  improve  reliability.  But  in 
practice,  very  few  items  of  "off-the-shelf" 
hardware  were  available.  An  instrument 
identical  to  one  from  an  earlier  mission 
usually  had  to  have  significant  design  changes 
to  work  on  a  new  spacecraft.  Most  important, 

redesign  is  often  necessary  because  manufac- 
turers no  longer  make  an  "old"  instrument's 
parts.  Some  instrument  redesign  was  necessary 
for  an  even  simpler  reason:  Pioneer  Venus 
was  NASA's  first  attempt  to  study  another 
planet's  atmosphere. 

An  example  was  the  Orbiter  electric-field 
detector.  Because  it  had  flown  on  earlier 
Pioneer  spacecraft,  mission  planners  thought 
it  might  fly  without  change  on  Pioneer  Venus. 
But  when  engineers  took  a  closer  look,  they 
realized  they  had  to  redesign  the  small  ball- 
like  antennas  on  the  detector.  As  it  turned 
out,  the  electric-field  detector  was  not  the 
only  equipment  to  be  redesigned.  Engineers 
did  a  lot  of  redesigning,  particularly  for  the 
Multiprobe,  because  no  spacecraft  like  the 
Multiprobe  had  ever  flown.  Engineers  were 
challenged  to  closely  package  instruments 
that  would  take  many  measurements  never 
before  taken.  Consequently,  many  instruments 
were  new  designs  that  involved  critical 
development  tasks. 

From  the  beginning,  the  neutral  mass  spec- 
trometer for  the  Large  Probe  was  the  most 
difficult  new  design.  Mission  planners  initially 
selected  more  instruments  than  they  would 
use.  The  neutral  mass  spectrometer  was  a 
prime  example.  Two  mass  spectrometers  were 
under  development — one  at  Goddard  Space 
Flight  Center  and  the  other  at  the  University 
of  Texas  at  Dallas.  Both  had  funds  for  a  year  of 
continued  in-house  development.  An  instru- 
ment review  committee  reviewed  all  instru- 
ments, in  particular  the  two  mass  spectrometer 
designs.  Eventually,  the  NASA  Headquarters 
Science  Steering  Committee  chose  the  Univer- 
sity of  Texas  instrument. 

Planners  chose  two  other  instruments  that  did 
not  fly.  One  was  a  radar  altimeter  for  the  Large 
Probe.  After  a  year's  work,  it  became  clear  that 


the  instrument  was  too  heavy,  too  complex, 
and  too  costly.  Since  scientists  could  derive 
altitude  as  a  function  of  time  from  the  probe's 
atmospheric  structure  experiment,  NASA 
decided  to  remove  the  radar  altimeter  experi- 
ment. The  other  instrument  was  a  photometer 
system  from  the  University  of  Wisconsin.  After 
a  year  of  in-house  study  at  the  University, 
NASA  Headquarters  realized  that  the  experi- 
ment was  neither  required  nor  far  enough 
along  for  the  mission. 

Early  in  the  program,  NASA  made  preliminary 
choices  about  experiments  and  then  amended 
them  as  more  information  became  available. 
There  was  nothing  unusual  about  preliminary 
selection  of  experiments  and  then  making  a 
final  selection  some  12  or  18  months  after- 
ward. For  example,  mission  scientists  never 
intended  both  mass  spectrometers  to  fly  on 
the  Multiprobe.  Also,  they  eliminated  the 
photometer  and  radar  altimeter  on  the 
grounds  of  payload  weight  and  development 
studies.  The  Orbiter,  however,  was  a  different 
story.  Mission  planners  approved  for  flight  all 
instruments  they  had  initially  selected  for  it. 

Challenges  of  Instrument 
Development:  Probes 

Many  mission  experiments  were,  indeed, 
unique.  The  big  problem  for  the  probes  was 
packing  all  the  instruments  into  a  small 
pressure  shell.  The  shell  protected  the  instru- 
ments as  they  traveled  through  Venus'  hostile 
environment.  For  the  Orbiter,  the  most 
difficult  task  was  ensuring  reliability  of 
operation  for  at  least  243  Earth  days. 

The  challenge  in  space  missions  is  always 
meeting  the  scheduled  launch  date.  For 
Pioneer  Venus,  all  the  instruments  were  ready 
on  time.  At  one  point,  however,  the  Jet  Propul- 
sion Laboratory  (JPL)  encountered  significant 
development  problems  with  the  infrared 

radiometer.  Within  a  year  of  the  launch  date, 
mission  planners  were  still  concerned  that 
they  might  have  to  scratch  the  instrument 
from  the  payload.  JPL  responded  by  intensify- 
ing its  development  effort  and  was  able  to  test 
the  completed  instrument  on  time. 

The  neutral  mass  spectrometer  was  a  principal 
development  challenge.  One  main  difficulty 
was  to  develop  an  inlet  system  for  the  instru- 
ment. While  most  mass  spectrometers  in  space 
applications  operate  under  quasi-vacuum  con- 
ditions, the  Pioneer  Venus  instrument  had 
to  operate  at  pressures  100  times  Earth's 

The  ion  source  of  every  mass  spectrometer  has 
to  operate  within  a  narrow  range  of  pressures. 
Therefore,  the  pressure  within  the  instrument 
has  to  remain  constant.  Engineers  needed  an 
inlet  system  that  would  reduce  the  ambient 
pressure  from  104  torr  to  the  10-5  torr  (106  pa 
to  10-3  pa)  that  the  ion  source  required.  This 
was  a  tremendous  pressure  reduction.  To 
achieve  this  reduction,  engineers  had  to  build 
the  inlet  system  to  admit  very  small  quantities 
of  gas.  Yet  these  small  quantities  had  to  be 
large  enough  for  analysis  before  the  instru- 
ment purged  itself  for  the  next  sample.  The 
University  of  Texas  designed  an  innovative 
system.  It  consisted  of  a  ceramic  microleak 
(CML)  inlet  and  a  variable  conductance  valve. 
Their  design  challenge  was  to  change  the 
instrument's  conductance  automatically.  Their 
novel  solution:  let  the  ambient  pressure  of  the 
Venusian  atmosphere  control  the  valve. 

When  engineers  attempted  to  adapt  the  CML 
for  the  Pioneer  Venus  mission,  they  ran  into 
snags.  Initially  the  inlet  was  stainless  steel. 
When  engineers  tested  it  in  sulfuric  acid 
vapor,  the  acid  never  entered  the  instrument 
for  sampling.  Instead,  it  became  trapped  in  the 
oxide  coating.  Since  one  mission  task  was  to 



check  for  acids  in  Venus'  atmosphere,  mission 
engineers  had  to  correct  this  problem.  It  took 
two  years  to  solve  the  problem.  They  devel- 
oped a  suitable  ceramic  coating  with  a  passi- 
vated  surface,  and  the  inlet  was  made  of 
tantalum  instead  of  stainless  steel. 

Scientists  realized,  too,  that  aerosol  particles 
in  the  planet's  atmosphere  might  block  the 
mass  spectrometer's  small  inlet  opening.  The 
University  of  Texas  developed  a  narrow  slit 
design  to  minimize  blockages,  and  engineers 
installed  a  heater  coil  around  the  inlet  to 
vaporize  such  particles.  (Despite  their  efforts, 
sulfuric  acid  droplets  covered  the  inlet  for  a 
time  during  the  mission  and  blocked  the  inlet.) 

The  mass  spectrometer  caused  even  more 
difficulties.  A  single  inlet  would  be  fine  in  the 
dense  lower  atmosphere.  But  in  the  upper 
rarefied  atmosphere,  this  instrument  needed 
an  additional  inlet  to  provide  sufficient  gas 
input.  Engineers  designed  the  second  inlet  to 
stay  open  until  roughly  the  time  of  parachute 
release.  Then  a  pyrotechnic  device  crushed  the 
line  and  stopped  further  gas  entry.  Even  if  the 
cutoff  device  failed,  there  would  still  be  a  valid 
set  of  data,  although  somewhat  degraded. 

In  addition  to  its  novel  inlet  design,  this 
instrument  had  several  other  firsts.  It  was  the 
first  mass  spectrometer  of  its  size  to  survive  the 
forces  from  an  entry  deceleration  of  400  g.  It 
also  used  the  first  microprocessor  to  fly  in 
space:  an  Intel  4004.  The  microprocessor 
allowed  the  spacecraft  to  take  a  full  spectrum 
of  data  once  every  minute  over  the  whole  mass 
range  of  200  amu.  The  microprocessor  selected 
the  true  data  point  from  several  data  points 
and  adjusted  for  calibration  changes.  A  high 
confidence  factor  was  associated  with  the 
single  data  point  transmitted.  Without  the 
microprocessor,  it  would  have  been  possible  to 
transmit  a  spectrum  only  once  for  every 

10-km  change  in  altitude.  With  the  micropro- 
cessor, sampling  occurred  at  every  1-km 
change  in  altitude. 

Other  instruments  also  posed  some  problems. 
For  example,  mission  scientists  had  not  origi- 
nally proposed  to  fly  a  gas  chromatograph. 
However,  the  original  study  team  developed 
strong  arguments  in  favor  of  the  gas  chro- 
matograph, and  they  finally  included  it  in  the 
payload  package.  (A  gas  chromatograph  is  a 
high-pressure  instrument  while  the  mass 
spectrometer  is  a  low-pressure  instrument.) 

At  Ames  Research  Center,  Vance  Oyama  had 
developed  a  gas  chromatograph  for  the  Viking 
landings  on  Mars.  Engineers  used  his  experi- 
ence to  design  an  instrument  for  Pioneer 
Venus.  In  the  program's  early  days,  mission 
planners  considered  the  chromatograph  a 
backup  instrument.  They  would  use  it  to  pro- 
vide some  spectra  of  atmospheric  composition 
if  the  mass  spectrometer  failed.  However,  they 
soon  saw  that  the  two  instruments  comple- 
mented each  other.  For  example,  the  gas  chro- 
matograph could  measure  water  vapor  that  the 
mass  spectrometer  could  not  measure  reliably. 

Robert  Knollenberg,  a  cloud  physicist,  had 
developed  a  small  spectrometer  that  the  U.S. 
Air  Force  used  to  measure  the  number  of  ice 
particles  in  clouds.  Knollenberg  and  Ball 
Brothers  Research  (Boulder,  Colorado)  adapted 
this  instrument  for  Pioneer  Venus.  Their  Cloud 
Particle  Size  Spectrometer  was  essentially  an 
optical  bench  with  a  laser  at  one  end  and  a 
prism  at  the  other.  Part  of  the  optical  bench 
had  to  be  outside  the  pressure  hull  of  the 
spacecraft.  This  design  had  a  drawback.  It 
exposed  the  bench  to  twisting  and  other  dis- 
tortions that  would  occur  as  the  pressure  vessel 
heated  in  Venus'  atmosphere.  Lou  Polaski, 
Ames  Research  Center,  was  responsible  for 
developing  probe  instruments.  After  studying 



the  problem,  he  realized  how  to  correct  the 
problem.  The  pressure  vessel  needed  heaters 
on  the  window  in  the  vessel  and  on  the  prism 
outside  the  window. 

Hughes  Aircraft  Company  was  the  contractor 
for  the  spacecraft  and  probes.  Hughes  also 
built  the  faceplate  through  which  the  optical 
bench  would  penetrate  the  probe's  wall.  To 
this  faceplate,  Hughes  attached  the  instrument 
parts  that  Ball  Brothers  supplied.  Ball  Brothers 
then  aligned  the  complete  unit.  Said  Polaski: 
"It  was  a  tremendous  challenge  to  get  a  very 
precise  optical  bench  through  a  wall  that 
was  changing  relative  to  the  rest  of  the 
optical  bench.  The  instrument  really  worked 
well  but  only  as  a  result  of  a  lot  of  good 
engineering  work." 

Another  unique  instrument  the  probe  carried 
was  a  solar  flux  radiometer.  It  was  unique 
because  engineers  developed  the  sensor 
portion  separately  from  all  the  electronics. 
Martin  Marietta,  Denver,  built  the  electronics. 
The  University  of  Arizona's  Optical  Science 
Center  designed  and  built  the  optical  head 
with  the  sensors. 

The  infrared  radiometer  used  warm  infrared 
detectors  that  had  to  remain  at  a  constant 
temperature.  To  maintain  this  temperature, 
the  detectors  were  packaged  in  phase-change 
material  (the  "blue  ice"  in  recreational 
refrigeration).  Technically,  this  material  is  a 
eutectic  salt.  The  gas  chromatograph  also 
controlled  the  temperature  of  its  columns  with 
"blue  ice."  To  control  the  temperature  of  its 
optical  head,  the  solar  flux  radiometer  used  it, 
too.  Scientists  picked  salts  that  would  keep  the 
temperature  at  the  required  value  (like  ice 
floating  in  water  will  keep  the  water  at  a 
constant  temperature  until  all  the  ice  has 
melted).  But  it  was  not  that  simple.  All  salts 
had  to  be  frozen  before  the  probe  entered  the 

Venus  atmosphere.  Mission  planners  had  to 
prove  conclusively  that  from  the  probe's 
release  from  the  Bus  to  its  arrival  at  the  Venus 
atmosphere — about  three  weeks — the  phase- 
change  material  would  remain  frozen.  That 
proof  took  considerable  time  and  effort. 

The  net  flux  radiometer  that  flew  on  each 
Small  Probe  had  a  flux  plate  that  flipped  back 
and  forth  to  measure  the  up  and  down  flux. 
This  radiometer  required  a  diamond  window 
that  was  smaller  than  the  Large  Probe's 
infrared  radiometer  window.  Two  diamond 
windows  were  on  each  side,  and  the  radiom- 
eter hung  out  over  the  back  of  the  probe.  Its 
strange  appearance  earned  it  the  nickname  of 
"The  Lollipop."  The  diamond  windows  came 
from  the  same  stone  as  the  big  window, 
ensuring  identical  infrared  transmission 
characteristics.  They  also  made  data  correla- 
tion between  the  two  instruments  easier. 
Seven  diamonds  thus  traveled  to  Venus — two 
diamond  windows  for  each  of  three  Small 
Probes  and  a  single  large  window  in  the 
Large  Probe. 

Challenge  of  Instrument 
Development:  Orbiter 

For  the  Orbiter,  the  most  significant  instru- 
ment under  development  was  the  radar 
mapper.  Hughes  Aircraft,  Culver  City,  Califor- 
nia, built  the  mapper  with  a  team  led  by 
Gordon  Pettengill  of  Massachusetts  Institute 
of  Technology.  The  complete  instrument  used 
more  than  1,000  microcircuits,  weighed  only 
about  10.9  kg  (24  Ib),  and  consumed  a  mere 
30  W.  This  was  the  first  time  engineers  had 
assembled  a  complex  instrument  for  radar 
mapping  in  such  a  compact  package.  The 
responsible  project  engineer  at  Ames  Research 
Center  was  Carl  Keller,  who  played  a  key  role 
in  the  instrument's  development. 



The  imaging  system  aboard  the  Orbiter  was  a 
second  generation  imaging  photopolarimeter 
that  flew  on  the  Pioneer  spacecraft  to  Jupiter 
and  Saturn.  For  the  Pioneer  Venus  mission, 
scientists  fitted  it  with  an  improved  telescope 
and  a  new  interface.  The  plasma  analyzer  also 
was  an  outgrowth  of  past  programs. 

The  overall  program  cost  for  instrument  devel- 
opment was  within  estimates.  Some  instru- 
ments ran  over  budget  because  problems  were 
met  in  development.  However,  others  came  in 
below  cost  because  problems  the  mission  bud- 
geted for  did  not  materialize.  Only  one  instru- 
ment was  very  late  in  delivery,  and  all  were 
ready  in  time  for  the  mission.  Despite  the 
instruments'  complexity,  the  mission's  finan- 
cial management  was  remarkable  in  control- 
ling costs  to  meet  budgets. 

Designing  the  Mission  and 
Developing  the  Spacecraft 

Paralleling  the  development  of  the  science 
payload,  the  project  had  been  busily  develop- 
ing the  spacecraft.  It  awarded  two  concurrent 
study  contracts  of  $500,000  each  on  October  2, 
1972.  One  contract  went  to  Hughes  Aircraft 
Company  Space  and  Communications  Group, 
teamed  with  General  Electric  Company.  The 
other  went  to  TRW  Systems  Group,  teamed 
with  Martin  Marietta.  The  contracts  called  for 
system  definition  by  June  30,  1973.  After  the 
contractors  defined  the  system,  NASA  would 
select  a  single  contractor  to  design,  develop, 
and  fabricate  the  spacecraft. 

The  two  contractors  took  different  approaches. 
TRW  considered  the  use  of  different  basic 
spacecraft  types  for  the  Multiprobe  Bus  and 
Orbiter.  Hughes  preferred  a  single  spacecraft 
design  that  would  serve  the  dual  purpose.  The 
probe  designs  of  the  two  contractor  teams 
were  similar  in  essentials,  although  the  Orbiter 
configurations  differed  significantly.  The  TRW 

design  aligned  the  Orbiter's  spin  axis  parallel 
to  the  plane  of  the  ecliptic  and  pointed  toward 
Earth.  The  fixed  high-gain  antenna  also 
pointed  to  Earth  like  the  TRW-built  Pioneer 
Jupiter/Saturn  spacecraft's  antenna.  In  this 
design,  several  instruments  were  mounted  on 
a  movable  platform  so  they  could  scan  the 
surface  of  Venus.  The  Hughes  design  had  the 
spacecraft's  spin  axis  perpendicular  to  the 
ecliptic  plane,  with  the  spin  of  the  spacecraft 
sweeping  the  field  of  view  across  Venus.  It 
also  was  to  despin  a  high-gain  antenna  and 
point  it  toward  Earth.  The  Hughes  design  won 
the  contract. 

Amid  the  challenge  of  solving  technical  prob- 
lems came  a  major  political  disappointment. 
Congress  did  not  authorize  a  mission  start  in 
the  1974  fiscal  year.  As  a  result,  it  was  not 
possible  to  meet  launch  dates  for  the  1976/77 
Multiprobe  mission.  At  this  point,  August 
1972,  mission  officials  changed  the  launch 
series.  They  planned  two  launches,  and  both 
would  fall  back  to  the  next  launch  opportu- 
nity. Both  the  Multiprobe  and  the  Orbiter 
would  use  launch  opportunities  in  1978  and 
arrive  at  Venus  about  the  same  time,  near  the 
end  of  1978. 

Overview  of  the  Mission 

The  two  Pioneer  flights  to  Venus  were  to 
explore  the  atmosphere  of  the  planet,  to  study 
its  surface  using  radar,  and  to  determine  its 
global  shape  and  internal  density  distribution. 
The  Orbiter  would  operate  for  eight  months  or 
more,  making  direct  and  remote  sensing 
measurements.  NASA  designed  the  Multiprobe 
spacecraft  to  separate  into  five  atmospheric 
entry  craft  some  12.9  million  km  (8  million 
miles)  before  reaching  Venus.  Each  probe  craft 
would  measure  characteristics  of  the  atmo- 
sphere from  its  highest  regions  to  the  surface 
of  the  planet.  These  measurements  would 
occur  in  periods  of  a  little  more  than  two 

hours  at  points  spread  over  the  planet's  Earth- 
facing  hemisphere. 

In  celestial  mechanics,  there  are  two  classifica- 
tions of  transfer  ellipse  trajectories  for  travel- 
ing between  planets.  A  trajectory  that  carries 
a  spacecraft  less  than  180°  around  the  Sun  on 
a  voyage  from  one  planetary  orbit  to  another 
is  a  Type  I  trajectory.  One  that  travels  more 
than  180°  is  a  Type  II  trajectory. 

For  Pioneer  Venus,  navigators  wanted  the 
Orbiter  to  fly  a  Type  II  trajectory  to  reduce  its 
velocity  upon  arrival  at  Venus.  As  a  result,  the 
spacecraft  would  need  much  less  propellant 
to  slow  it  into  an  orbit  around  Venus — about 
180  kg  (400  Ib)  of  propellant  out  of  the  space- 
craft's total  weight  of  545  kg  (1200  Ib).  A  Type  I 
trajectory  to  Venus  would  have  required  50% 
of  the  total  spacecraft  weight  to  be  propellant. 
The  plan  had  the  Orbiter  launch  during  the 
period  May  20  through  June  10,  1975.  It  would 
follow  a  seven-month  flightpath  to  Venus 
along  a  trajectory  of  about  480  million  km 
(300  million  miles)  (Figure  2-3).  The  long  trajec- 
tory would  reduce  both  the  propellent's  weight 
and  the  orbital  insertion  motor's  weight  and 
size.  This  path  also  permitted  the  periapsis  to 
be  about  latitude  20°  N  on  the  planet. 

For  the  first  82  days  after  launch,  the  Orbiter 
spacecraft  would  fly  outside  Earth's  orbit.  It 
would  then  cross  Earth's  orbit  and  plunge 
inward  on  a  long  curving  path  toward  the 
Sun.  It  would  arrive  at  Venus  on  December  4, 
1975,  five  days  before  the  arrival  of  the  probes, 
which  would  follow  a  shorter  flightpath.  The 
Multiprobe  spacecraft  would  be  launched  a  few 
days  after  the  Orbiter  crossed  Earth's  orbit,  dur- 
ing August  7  through  September  3.  This  space- 
craft would  follow  a  shorter,  Type  I  trajectory. 

Observing  Venus  from  Orbit 

On  the  Orbiter's  arrival  at  Venus,  the  mission 
plan  called  for  the  spacecraft's  motor  to  thrust 
for  28  seconds.  This  was  the  first  time  mission 
controllers  would  use  a  solid-propellant  motor 
stored  in  the  vacuum  of  space  so  long  (125  days) 
for  an  orbit  insertion  maneuver.  Their  aim  was 
to  reduce  the  spacecraft's  velocity  so  it  would 
enter  an  elliptical  orbit  with  a  24-hour  period. 
The  orbit  was  oriented  75  degrees  to  the 
equator  of  Venus — somewhat  more  inclined 
than  the  January  1973  study  report  suggested. 
Navigators  initially  desired  a  periapsis  of  300  km 
(186  miles).  They  also  wanted  an  apoapsis  of 
66,000  km  (41,012  miles).  Later  they  would 
command  the  spacecraft  into  an  orbit  having  a 
periapsis  of  150  km  (93  miles).  The  orbit's 
eccentricity  and  inclination  would  accomplish 
a  variety  of  scientific  goals  and  meet  a  number 
of  engineering  requirements. 

The  periapsis  would  allow  a  remote  sounding 
radar  mapper  to  study  the  planet's  surface  and 
several  instruments  to  take  measurements 
within  the  upper  atmosphere  and  ionosphere. 
It  also  provided  excellent  viewing  geometry  for 
remote  sensing  atmospheric  experiments. 

The  apoapsis  would  produce  an  orbit  with  a 
period  of  about  24  hours,  which  was  beneficial 
to  tracking  and  ground  operations.  It  also 
produced  a  nearly  one-to-one  correspondence 
of  orbit  numbers  with  days  into  the  mission. 
For  science,  it  provided  good  viewing  geom- 
etry for  obtaining  cloud  images  and  a  wide 
sampling  region  for  solar  wind  interaction 

The  orbit's  high  inclination  permitted  the 
spacecraft  to  make  measurements  and  direct 
observations  over  a  wide  range  of  latitudes. 


Figure  2-3,  The  Orbiter's 
trajectory  carried  it  first  outside 
the  Earth 's  orbit  for  nearly  half 
of  its  journey  to  Venus.  This 
trajectory  minimized  the  amount 
of  propellant  needed  to  enter 
orbit  around  Venus. 


Perturbation  of  the  spacecraft's  orbit  by  solar 
gravity  would  change  significantly  the  altitude 
and  latitude  of  the  periapsis.  For  the  first 
20  months  of  the  mission  (Phase  I),  flight  con- 
trollers would  use  the  spacecraft's  thrusters  to 
counteract  altitude  drift.  Later,  during  Phase  II, 
they  would  allow  periapsis  to  rise  to  a  maxi- 
mum of  2310  km  (1435  miles)  by  July  1986. 
At  that  time,  solar  perturbation  would  cause 
the  periapsis  to  descend  again.  In  the  same 
period,  the  latitude  of  periapsis  would  move 
from  1 7°  N  to  the  equator. 

Phase  III  would  begin  in  late  1991  as  periapsis 
again  reached  the  lower  thermosphere  and 

ionosphere.  Mission  planners  arbitrarily 
defined  the  transition  from  Phase  II  to  Phase  III 
to  take  place  when  the  periapsis  altitude 
reached  1000  km  (621  miles).  In  1992,  flight 
controllers  would  use  the  remaining  hydrazine 
propellant  to  keep  the  periapsis  within  140  to 
160  km  (87  to  100  miles).  During  Phase  III,  the 
Orbiter  would  penetrate  and  sample  deeper  in 
the  atmosphere  than  was  acceptable  during 
Phase  I.  Phase  III  would  end  in  October  1992 
when  the  hydrazine  propellant  was  exhausted 
and  the  spacecraft's  periapsis  had  descended 
into  the  atmosphere.  By  that  time,  drag  would 
pull  the  spacecraft  from  orbit  into  a  meteoric 
ending  of  a  14-year  mission. 

Probing  Venus'  Atmosphere 

The  Multiprobe's  four-month  trip  to  Venus 
resulted  in  the  spacecraft  approaching  the 
planet  at  or  about  1900  km/hr  (1180  mph). 
The  comparative  trajectories  for  the  Orbiter 
and  the  Multiprobe  appear  in  Figure  2-4. 

as  simple  as  possible,  they  decided  on  a 
simultaneous  launch  from  the  Bus.  In  a 
one-firing  episode,  they  could  release  all  three 
Small  Probes;  separate  launches  would  have 
been  less  reliable.  However,  this  single  launch 
episode  demanded  detailed  computer  analysis. 

Orbiter  launch 
May /June  1978 

Venus  at  Orbiter 

Venus  at  Probe 

Probe  launch 
August  1978 

Venus  at  Probe 


December  1978 

Orbiter  arrives 
December  1978 

Probe  release 

Earth  at 



Earth  at  Probe 

Figure  2-4.  The  Mult/probe 
followed  a  shorter  trajectory  and 
arrived  at  Venus  a  few  days  after 
the  Orbiter.  This  drawing 
compares  the  two  trajectories. 

Twenty-four  days  before  the  probes  entered 
Venus'  atmosphere,  the  Multiprobe's  axis 
would  lie  along  the  trajectory  that  the  Large 
Probe  would  follow  to  Venus.  The  probe  was 
then  launched  to  follow  its  own  path  to  the 
planet.  Next,  flight  controllers  changed  the 
Bus'  path  to  point  toward  the  center  of  Venus. 
This  change  allowed  the  Small  Probes  to  leave 
the  spinning  Bus  when  it  was  20  days  from 
the  planet.  The  spin  insured  that  the  Small 
Probes  separated  along  paths  that  would 
take  them  to  their  individual  targets  on  the 
planet  (Figure  2-5). 

Originally,  mission  planners  had  discussed  an 
alternative  concept.  In  this  scenario,  they 
would  have  individually  targeted  the  three 
Small  Probes  and  each  would  have  separated 
individually  from  the  Bus.  To  keep  the  system 

Where  should  the  spin  axis  be  pointed?  At 
what  spin  rate  should  the  spacecraft  operate 
for  the  release?  Mission  planners  needed  these 
answers  so  they  could  direct  the  probes  to 
enter  Venus'  atmosphere  near  the  planet's 
limb  regions  (as  viewed  from  Earth).  But  the 
entry  could  not  be  too  close  to  the  limb  to 
limit  slant-range  communications  through  the 
planet's  atmosphere.  A  computer  program 
displayed  different  targeting  options.  Specifi- 
cally, the  program  determined  the  angle  of 
attack  of  each  probe's  entry  into  the  Venusian 
atmosphere.  All  the  probes  were  stabilized  by 
their  rotation,  but  what  would  happen  if 
one  entered  the  atmosphere  sideways?  Its 
heat  shield  would  not  have  protected  it  from 
the  heat  of  entry.  The  probe  would  have 
been  destroyed. 


Figure  2-5.  Approaching  Venus, 
the  Multiprobe  released  its  four 
probes  toward  different  target 
areas  on  the  planet.  (Top) 
Artist's  concept  of  the  probes 
and  the  Bus  shortly  after  their 
release.  (Bottom)  Diagram  of  the 
paths  of  the  probes  and  their 
entry  points  on  the  planet  in 
relation  to  the  orbit 
of  the  Orbiter  spacecraft. 



Night        reverse  side 

On  arrival  at  Venus,  the  four  probes  entered 
the  atmosphere.  The  Large  Probe  took  about 
55  minutes  to  descend  to  the  surface,  the 
three  Small  Probes,  about  57  minutes.  None 
of  the  probes  were  designed  to  withstand 
impact  with  the  surface,  each  hitting  it  at  about 
36  km/hr  (22  mph).  The  Bus  itself  hurtled  into 
the  upper  atmosphere  about  80  minutes  after 
the  probes.  Unlike  the  probes,  the  Bus  carried 
no  heat  shield;  its  task  was  to  provide  data 
only  on  the  atmosphere's  highest  part. 

All  probes  sent  their  data  directly  to  Earth  as 
they  penetrated  Venus'  atmosphere  on  the 
hemisphere  that  faced  Earth. 

Launch  Vehicle 

Originally,  the  project  planned  to  use  the 
Thor-Delta  launch  vehicle  for  the  Pioneer 
Venus  flight.  The  system  definition  studies 
began  with  this  launch  capability  as  a  design 
criterion  for  the  two  spacecraft.  However,  very 
early  in  the  study  it  became  clear  that  costs 
were  rapidly  rising  as  subsystem  designs  were 
severely  restricted  in  weight  and  size.  To 
reverse  this  trend,  the  project  team  asked 
competing  contractors  to  study  an  alternative 
design  that  removed  the  weight  and  size 
restrictions.  The  contractors  did  this  by 
comparing  design  and  cost  estimate  results  of 
an  Atlas-Centaur  launch  with  the  launch 
capabilities  of  the  Thor-Delta. 

Based  on  these  analyses,  mission  planners 
determined  that  the  additional  $10  million  for 
the  Atlas-Centaur  launch  vehicle  would  be 
acceptable.  (The  costs  would  at  least  equal  the 
increased  costs  required  to  cover  the  miniatur- 
ization of  the  Multiprobe  and  Orbiter  space- 
craft designs  for  the  Thor-Delta  requirements.) 
NASA,  therefore,  approved  use  of  the  Atlas- 
Centaur — NASA's  standard  launch  vehicle  for 

payloads  of  intermediate  weight  (Figure  2-6). 
The  Atlas-Centaur  launch  vehicle  stands 
about  40  m  (131  ft)  high.  It  consists  of  an  Atlas 
SLV-3D  booster  with  a  Centaur  D-1A  second 
stage.  The  nation's  first  high-energy  launch 
vehicle,  Atlas-Centaur  used  liquid  hydrogen 
and  liquid  oxygen  propellants  for  its  upper 
Centaur  stage.  Engineers  enclosed  each 
spacecraft  in  a  fiberglass  nose  fairing  to 
protect  it  as  the  launch  vehicle  sped  through 
Earth's  atmosphere. 

"New  Start"  Approved 
for  Fiscal  Year  1975 

By  July  1973,  the  system  definition  studies 
were  completed  and  each  team  received  a 
holding  contract.  The  next  step  involved 
competitive  bidding  following  issue  of  a 
Request  for  Proposal  in  June  1973.  Based  on 
the  bidding  results,  the  project  team  selected 
Hughes  Aircraft  Company  in  February  1974 
to  negotiate  a  cost-plus-award-fee  (CPAF) 


Figure  2-6.  An  Atlas-Centaur, 
NASA's  standard  launcher  for 
payloads  of  intermediate  weight, 
carried  each  of  the  Pioneer  Venus 


contract.  This  contract  covered  the  initial 
conceptual  design  phase  of  the  system.  The 
proposed  cost  of  design  work  for  this  phase 
was  $3  million.  There  also  was  an  option  for 
final  design,  development,  fabrication,  testing 
of  two  flight  spacecraft,  and  launch  support  at 
$55  million.  NASA  awarded  a  contract  in  May 

1974,  but  not  for  the  hardware.  The  mission 
still  waited  for  Congressional  approval  of  a 
"new  start" — a  new  authorized  space  mission 
for  fiscal  year  1975. 

In  August  1974,  Congress  finally  approved  a 
new  start  for  Pioneer  Venus  for  fiscal  year 

1975.  Further  negotiations  took  place  with  the 
contractor.  In  November  1974,  NASA  made  a 
final  award,  including  hardware,  to  Hughes 
Aircraft  Company. 

System  specifications  were  completed  by 
February  1975.  By  the  beginning  of  calendar 
year  1975,  work  was  well  under  way.  But  the 
program  still  had  to  face  major  hurdles  before 
launch.  Said  Charles  Hall,  project  manager:  "It 
always  seems  you  don't  have  enough  time  and 
you  are  trying  to  find  ways  to  do  things  faster. 
You  are  always  having  trouble  with  funding. 
You  may  have  a  total  amount  of  funds  that  is 
enough  for  the  program  but  you  never  seem  to 
have  enough  for  any  particular  year.  So  you 
are  always  making  small  perturbations  to  your 
plans  to  work  around  funding  difficulties." 

New  Funding  Problems 

In  June  of  1975,  during  the  budget  hearings 
for  fiscal  year  1976,  Pioneer  Venus  suffered  a 
serious  setback.  The  House  of  Representatives 
voted  to  cut  $48  million  from  the  NASA 
appropriations  for  the  Venus  mission.  NASA 
had  already  spent  $50  million  on  the  program. 
The  House  vote  was  based  on  misinformation 
and  a  lack  of  understanding  about  the  techni- 
cal problems  associated  with  a  delay.  Suppose 
NASA  had  delayed  the  launch  to  the  1980 

opportunity  (the  most  likely  scenario  if 
Congress  had  withheld  funds).  What  would 
have  happened?  Engineers  would  have  had 
to  redesign  the  spacecraft  because  the  1980 
launch  opportunity  was  not  as  favorable  as 
1978.  More  launch  energy  or  a  lesser  payload 
would  have  been  the  result.  That  might  have 
been  the  end  of  the  program.  NASA  would 
have  needed  as  much  as  $50  million  extra 
(over  what  they  originally  requested)  for 
a  mission  to  Venus  at  the  less  favorable 
launch  opportunity. 

However,  scientists,  the  national  press,  and 
many  organizations  rallied  to  Pioneer  Venus. 
Important  scientific  groups  lent  their  support. 
The  Nation's  most  famous  climatologists  and 
meteorologists  stressed  the  importance  of 
more  and  better  information  about  the 
weather  and  climate  of  Venus  and  Mars. 
Increases  in  the  world's  population  make  it 
increasingly  important  that  we  understand 
Earth's  climate  better.  We  all  would  benefit  by 
an  ability  to  predict  accurately  long-term 
changes  that  might  lead  to  droughts  and  poor 
harvests.  Scientists  from  many  fields  pointed 
out  the  mission's  importance  to  Earth  sciences 
and  to  finding  ways  to  lessen  the  effects  of 
climate  changes  on  food  production.  (For 
example,  one  scientist  pointed  out  that  a 
change  in  Earth's  average  temperature  of  only 
1.5°C  could  wipe  out  Canada's  entire  wheat 
production.  If  such  a  change  should  occur 
unexpectedly,  the  effects  on  world  food  sup- 
plies could  be  disastrous.)  Scientists  stressed 
that  understanding  weather  systems  on  Venus 
and  Mars  was  essential  to  a  better  understand- 
ing of  Earth's  weather  systems. 

A  Senate  subcommittee  restored  funds  for 
Pioneer  Venus  in  July  1975.  This  action 
reversed  the  House  move  to  slash  all  but 
$9.2  million  from  the  project.  But  NASA's 
worries  weren't  over,  yet.  The  project  still 

faced  high  hurdles.  The  Senate  Appropriations 
Committee  and  then  the  full  Senate  had  to 
approve  the  funds.  If  they  did,  a  joint  commit- 
tee would  still  have  to  work  out  a  compromise 
with  the  House.  The  Senate  committee  acted 
on  the  bill  later  in  July  and  gave  support  to  the 
mission.  Early  the  next  month,  the  Senate  also 
recognized  the  program's  importance.  The 
Senators  gave  their  approval  to  NASA's 
requested  funding  of  $57  million  for  Pioneer 
Venus  during  fiscal  year  1976. 

During  September  1975,  the  go-ahead  finally 
came.  The  Senate-House  conference  committee 
restored  all  but  $1  million  of  the  funds  to  send 
the  two  Pioneer  spacecraft  to  Venus  in  1978. 
The  Earth-based  part  of  the  mission  was  back 
on  course.  Scientists  and  engineers  could  again 
concentrate  on  their  main  task:  having  the 
spacecraft  and  their  scientific  instruments 
ready  for  launch  opportunities. 

Parachute  Development 

By  June  1975,  final  contracts  for  scientific 
instruments  were  ready  for  signing.  By  July, 
engineers  had  studied  and  resolved  most 
problems  of  integrating  instruments  into  the 
spacecraft.  The  first  tests  of  the  parachute 
system,  needed  for  the  descent  of  the  Large 
Probe  into  the  Venusian  atmosphere,  had 
started.  This  aspect  of  the  Pioneer  Venus  pro- 
gram made  use  of  the  largest  structure  of  its 
type  in  the  world:  the  Vertical  Assembly  Build- 
ing at  NASA's  Kennedy  Space  Center,  Florida. 
(NASA  originally  built  the  structure  for  final 
assembly  of  the  huge  Saturn  V  boosters  that 
launched  Apollo  spacecraft  to  the  Moon.) 
NASA  used  the  building  to  test  the  Large 
Probe's  parachute.  In  the  test  series,  engineers 
dropped  full-size  parachutes  with  pressure 
vessels  of  various  weights  135  m  (443  ft)  in  the 
wind-free  environment  of  the  building.  The 
series  helped  them  determine  the  parachute's 
aerodynamic  trim  characteristics  (Figure  2-7). 

The  Large  Probe  parachute  was  an  important 
development  item.  It  was  essential  because  it 
would  delay  the  descent  of  the  Large  Probe 
long  enough  to  make  many  measurements  as 
it  settled  through  the  clouds. 

"For  a  time  it  almost  looked  as  though  we  were 
never  going  to  get  a  parachute,"  commented 
Charles  Hall  after  the  mission.  He  related  how 
they  had  taken  a  newly  designed  parachute  to 
the  desert  near  El  Centre,  California,  for  a 
drop  test  from  an  F-4  airplane.  Personnel 
attached  the  parachute  to  a  pointed  cylinder 
that  carried  high-speed  (200  frames/sec) 
cameras  and  test  instruments.  When  the 
airplane  was  traveling  at  high  speed  and 
proper  altitude,  the  cylinder  would  drop  and 
the  parachute  deploy,  a  drogue  chute  pulling 
out  the  main  chute. 

The  day  of  the  test  arrived.  As  everyone  had 
expected,  the  cylinder  dropped  and  the  drogue 
chute  deployed.  Observers  were  appalled,  how- 
ever, to  see  no  trace  of  the  main  chute  opening. 

Figure  2-7.  The  parachute  for 
the  Large  Probe  was  tested 
initially  in  drop  tests  within  the 
large  Vertical  Assembly  Building 
at  NASA's  Kennedy  Space 
Center,  Florida. 



It  literally  disappeared.  Hall  described  how, 
when  investigators  examined  the  film  records, 
they  found  the  parachute  starting  to  open  and 
then  disintegrating  into  shreds.  The  camera 
speed  was  200  frames/sec,  and  they  could  view 
the  film  one  frame  at  a  time.  Said  Hall:  "You 
wouldn't  believe  it,  but  on  one  frame  the 
parachute  would  be  intact  and  on  the  next 
frame  there  would  be  nothing  there.  It  was  not 
that  it  was  breaking  away  from  the  shrouds, 
the  material  itself  was  just  ripped  to  shreds." 

Engineers  thought  that  the  test  environment 
was  too  severe.  So  they  planned  another  test  in 
which  a  lower  dynamic  pressure  was  exerted 
on  the  parachute.  The  results  were  equally  bad. 

A  third  try  also  failed.  But  Hughes  engineers 
inspected  the  pictures  more  closely.  They 
noticed  that  when  the  parachute  was  still 
intact,  in  the  frame  just  before  complete  fail- 
ure, many  of  the  parachute  gores  (the  angular 
sections  of  the  parachute)  were  missing, 
although  the  chute  fully  deployed.  They 
suspected  this  was  the  cause  of  the  trouble 
since  the  part  that  opened  would  experience 
greater  stresses  than  its  design  allowed. 

Engineers  next  deployed  one  of  the  parachutes 
in  Ames  Research  Center's  40-  by  80-Foot 
Wind  Tunnel.  Even  there  all  the  gores  did  not 
open.  The  low  wind  speed  in  the  tunnel  was 
then  reduced  to  a  relative  breeze  so  an  engi- 
neer could  walk  inside  and  watch  the  opening. 
When  the  parachute  opened  and  the  gores 
still  stayed  folded,  he  tried  to  pull  them  apart 
but  could  not.  The  chute's  design  allowed  the 
wind  load  to  effectively  hold  the  gores 
together.  As  a  result,  NASA  abandoned  this 
parachute  design  in  favor  of  an  earlier  conical 
ribbon  design. 

Time  was  running  out,  and  NASA  had  to  take 
some  chances.  When  the  new  parachute  was 

ready,  engineers  put  it  through  a  final  system 
drop  test.  There  was  not  even  time  to  try  it 
with  airplane  drops  first.  In  the  earlier  tests, 
the  falling  body  had  not  been  a  sphere  with  a 
heat  shield.  Because  of  time  constraints,  NASA 
had  to  test  the  parachute,  the  heat  shield 
release  mechanism,  and  other  hardware  on 
one  drop  from  a  high-altitude  balloon. 

In  December  1976,  mission  engineers  tested 
the  system  in  a  balloon  drop  at  the  Army's 
White  Sands  Missile  Range  in  New  Mexico. 
The  parachute  deployed  at  an  altitude  of 

16  km  (10  miles).  At  that  altitude,  atmospheric 
temperature  and  density  and  the  probe's  speed 
would  be  close  to  the  conditions  on  Venus  just 
before  its  descent  into  that  planet's  dense,  hot, 
lower  atmosphere.  Personnel  designed  the 
tests  to  confirm  several  features:  deployment 
of  the  probe  parachute,  separation  of  the 
atmospheric  entry  heat  shield,  and,  after 

1 7  minutes  of  parachute  descent,  separation  of 
the  pressure  vessel  for  its  free-fall  plunge.  The 
fast  descent  after  the  parachute's  release  would 
let  the  probe  penetrate  deeply  into  the  Venu- 
sian  atmosphere.  This  plunge  would  be  com- 
pleted before  high  temperatures  could  destroy 
the  probe's  instruments. 

The  sky  was  clear  at  4:00  a.m.  when  the 
balloon  gently  lifted  its  load  from  White  Sands 
Proving  Grounds,  New  Mexico.  Ponderously, 
the  great  plastic  bag  carried  the  test  vehicle  to 
an  altitude  of  31  km  (19  miles).  At  Ames 
Research  Center,  project  leaders  waited  for  the 
test  results.  Says  Hall:  "We  got  the  phone 
call ...  'It  has  been  a  complete  failure.'"  When 
staff  gave  the  radio  command  to  release  the 
vehicle,  it  dropped  swiftly  from  the  gondola 
beneath  the  balloon,  just  as  mission  scientists 
had  planned.  However,  as  the  probe  released 
from  the  balloon,  it  hit  the  gondola,  which 
caused  the  probe  to  turn  upside  down.  Thus, 
when  the  parachute  released,  it  pulled  against 

the  parachute  clevises  in  the  wrong  direction 
and  broke  them.  The  test  vehicle  plunged  to 
the  desert  floor.  "We  were  in  trouble,"  Hall 
said.  "We  did  not  have  a  parachute." 

When  investigators  studied  the  photographs  at 
Ames  Research  Center  and  later  carefully 
inspected  recovered  parts  of  the  test  vehicle, 
they  discovered  the  reason  for  the  failure. 
There  were  structural  breakages  all  over  the 
test  vehicle.  And  these  breaks  had  all  occurred 
before  impact.  Obviously,  the  way  the  para- 
chute released  had  caused  the  structural 
damage.  At  first  it  seemed  that  not  only  had 
the  parachute  failed,  but  also  the  whole  system 
had  not  been  properly  stressed. 

Engineers  studied  the  photographs  in  detail. 
They  saw  that  the  test  vehicle  had  been 
tumbling  before  the  parachute  deployed  at 
18,000  m  (60,000  ft).  In  fact,  after  tumbling 
part  of  the  way  down,  the  test  vehicle  became 
stable,  but  fell  tail  first  instead  of  nose  first. 
When  the  parachute  deployed,  it  came  off  at 
an  angle  that  it  was  not  designed  for.  The  pic- 
tures showed  the  chute  being  deployed,  and, 
in  the  next  split  second,  the  chute  breaking 
away  from  the  body  of  the  Large  Probe  because 
of  its  wrong  attitude. 

Why  had  the  test  vehicle  tumbled  during  its 
fall  from  the  balloon?  For  the  journey  upward, 
it  had  been  in  a  container  about  3  m  square 
(10  ft  square).  At  the  last  minute,  a  test 
engineer  became  worried.  He  feared  that,  in 
the  gondola's  ascent  to  30,500  m  (100,000  ft), 
the  temperature  would  drop  too  low,  and 
equipment  in  the  Large  Probe  would  fail  to 
operate  correctly.  As  a  result,  he  taped  a 
protective  blanket,  made  of  1.3-cm  (0.5-in.) 
fibrous  padding,  beneath  the  box.  When  the 
probe  fell  through  the  blanket,  one  edge 
caught  on  the  blanket,  and  the  probe  tumbled 
in  its  fall. 

Engineers  built  another  test  vehicle,  made 
another  drop,  and  finally  achieved  success. 
Pioneer  Venus  finally  had  a  working  parachute 
for  its  Large  Probe. 

Spacecraft  Development  Challenges 

Test  engineers  suffered  anxious  moments 
during  a  thermal  vacuum  test  of  the  probe 
only  seven  months  before  the  launch  date. 
During  the  test,  the  batteries  within  the 
spacecraft  failed  completely.  With  the  launch 
date  so  close,  this  looked  like  a  major  disaster 
for  the  program. 

"In  retrospect,"  said  Charles  Hall,  "all  these 
things  look  simple,  but  at  the  time  we  had  no 
idea  whether  it  was  the  test  environment  or  the 
battery  at  fault.  We  made  many  side  tests  and 
had  experts  give  their  opinions,  and  as  is  gen- 
erally the  case  with  these  problems  you  can  get 
about  as  many  people  on  one  side  as  the  other." 

Investigation  showed,  however,  that  the 
batteries  themselves  were  not  at  fault.  It  was 
the  conditions  of  the  test  that  had  caused  the 
failure.  During  the  test,  the  spacecraft  spun  on 
an  axis  aligned  horizontally.  The  g  force  thus 
varied  in  direction  during  each  revolution. 
As  a  result,  the  electrolyte  sloshed  within  the 
batteries,  a  condition  that  would  not  occur 
during  an  actual  mission.  This  sloshing  caused 
massive  failures  within  the  battery's  cells. 

The  cable  connections  within  the  probes'  con- 
fined space  also  led  to  difficulties.  Within  a 
spacecraft,  the  cable  harness  nearly  always 
presents  problems.  This  was  especially  true  for 
the  Venus  probes.  The  harnesses  for  these 
probes  were  difficult  to  design  because  the 
probes  had  to  be  taken  apart  several  times  dur- 
ing testing.  Assembling  and  disassembling  the 
spacecraft  often  caused  testing  problems. 
Engineers  installed  equipment  on  two  shelves 
and  interconnected  them  by  the  harnesses.  The 


Figure  2-8.  Many  tests  were 
necessary  to  ensure  that  the 
probes  would  be  able  to  with- 
stand the  enormous  pressures 
and  high  temperatures  of  Venus ' 
atmosphere.  (Top)  The  pressure 
vessel  of  the  Small  Probe  appears 
partially  assembled  prior  to 
running  pressure  descent  tests  on 
it.  Engineers  machined  the  two- 
piece  structure  from  titanium 
forgings.  It  weighed  approxi- 
mately 18  kg  (40  Ib).  Its 
diameter  was  46  cm  (18  in.), 
and  the  wall  thickness  averaged 
approximately  0.3  cm  (1/8  in.). 
(Middle)  Engineers  constructed 
this  full-scale  mockup  of  the 
Large  Probe's  pressure  vessel 
module  during  Phase  B.  They 
used  it  to  study  the  packaging 
problems  inherent  in  spherical 
geometry.  (Bottom)  A  buckling 
indicator  mandrel,  used  during 
buckling  tests  of  probe  vessel 
scale  models,  appears  before 
assembly  with  a  test  model.  In 
each  test,  the  mandrel  supported 
the  inside  surface  of  the  pressure 
vessel  model  and  recorded  the 
imprint  of  the  buckle  pattern  on 
its  graphite  coated  surface.  This 
procedure  helped  determine 
where  failures  occurred. 


standard  procedure  with 
spacecraft  was  to  assemble 
the  whole  package  and  test 
it.  When  they  were  done, 
they  took  it  apart  again  so 
the  principal  investigators 
could  make  final  calibra- 
tions on  the  instruments  in 
their  laboratories.  They  had 
to  replace  instruments  in 
the  spacecraft  before  ship- 
ping it  to  the  launch  area 
for  mating  with  the  launch 

From  a  systems  integration 
and  test  standpoint,  the 
Multiprobe  Bus  and 
Orbiter's  common  features 
helped  simplify  testing 
problems  and  software 
development  for  the  test 

Another  major  problem  was 
sealing  the  probes.  On  the 
probes'  way  to  Venus, 
internal  pressure  had  to  be 
maintained  against  leakages  into  the  vacuum 
of  space.  When  a  probe  entered  Venus'  atmo- 
sphere, it  had  to  resist  the  tremendous  pressures 
and  prevent  inward  leaks.  During  development 
of  the  spacecraft,  engineers  made  many 
pressure  tests  (Figure  2-8).  They  wanted  to  make 
sure  that  the  titanium  shell  could  withstand 
the  pressure  and  that  the  seals  did  not  leak. 
Two  types  of  seals  were  necessary  for  these 
opposing  conditions.  They  both  required  a 
unique  design  and  many  more  tests  (Figure  2-9). 
For  the  vacuum  of  space,  an  O-ring  type  of  seal 
was  best.  To  resist  the  high  pressure  of  the 
Venusian  atmosphere,  engineers  used  flat 
Graphoil  seals  (made  of  graphite  fibers)  between 
flat  surfaces  on  flanges  of  the  spacecraft  parts. 

The  system 

worked  well.  One 

probe  actually 

sent  data  after  it 

had  landed  on  Venus'  surface,  and  these 

data  showed  no  evidence  of  any  damage. 

Sealing  the  various  spacecraft  windows  pre- 
sented another  series  of  problems.  Engineers 
made  many  tests  to  ensure  the  seals  would 
withstand  both  high  pressure  and  tempera- 
tures (Figure  2-10). 
Significant  develop- 
ment problems 
occurred,  however, 
in  making  a 
suitable  seal  for  the 
diamond  window 
(Figure  2-11). 
Engineers  decided 
early  not  to  braze 
the  window  to  the 
shell  of  the  pressure 
vessel.  Later,  they 
reversed  this  deci- 
sion and  decided  to  coat  the  edge  and  then 
braze  it  to  the  diamond.  As  the  program 
continued,  window  sealing  remained  a  very 
difficult  fabrication  problem.  In  fact,  it  became 
a  pacing  problem  that  prevented  testing  the 
flight  diamond  window,  with  its  full  assembly, 
with  the  instruments  in  the  spacecraft. 

There  were  many  more  disappointments  with 
the  windows.  Engineers  would  think  they  had 
a  solution,  but  when  they  tried  it,  it  would  fail. 
They  would  try  again,  but  just  when  they 
thought  they  had  completed  a  successful  test,  the 
window  seal  would  spring  another  leak.  Eventu- 
ally, they  had  to  use  a  mechanical  flat  seal. 

As  time  for  shipment  of  the  Large  Probe 
approached,  engineers  decided  that  the  inter- 
nal pressure  might  be  too  low  when  the  probe 

entered  Venus'  atmosphere.  To  increase  pres- 
sure by  6  psia,  they  decided  to  add  a  nitrogen 
pressure  bottle  to  the  payload.  This  nitrogen 
bottle  had  a  volume  of  110  cm3  (6.7  in.3) 
which,  with  the  nitrogen  stored  at  4,000  psia, 
would  increase  the  internal  pressure  by  6  psia. 
With  attachments,  the  bottle  added 

3.5  kg  (7.8  Ib)  to  the  Large  Probe's  weight.  An 
electrically  fired  squib  valve  punctured  a 
sealing  diaphragm  and  opened  the  bottle 
before  the  Large  Probe  entered  the  atmosphere. 
The  rate  of  release  was  5  psia/min.  This 
addition  required  wiring  changes.  At  the 
eleventh  hour,  the  squib  valve  did  not  puncture 

Figure  2-9.  A  metal  pressure  seal 
for  a  probe  pressure  vessel 
appears  with  its  disassembled 
testing  fixture.  The  seal  has 
undergone  a  sealing  test  in  a 
simulated  environment  that 
approximates  a  descent  through 
Venus '  atmosphere. 

Figure  2- 1 0.  (Top)  A  side  view  of 
diameter  sapphire  window 
assembly  after  a  test  in  a 
simulated  Venus  atmosphere 
pressure  and  temperature.  The 
assembly  consisted  of  an  Inconel 
housing  with  a  Kovar-sheathed 
heater  wrapped  around  a  brazed 
sapphire  window.  (Bottom) 
Result  of  pressure  testing  a 
sapphire  window  and  its  mount 
that  were  representative  of  the 
probe's  windows.  Note  that  the 
mount  failed  before  the  window 
itself  failed.  The  failure  pressure 
was  approximately  three  times 
the  maximum  amount  scientists 
expected  at  Venus '  surface. 


Figure  2-11.  This  photograph 
depicts  an  early  configuration  of 
the  diamond  window  and  its 
heater  assembly.  The  test  article 
incorporated  a  1 0-mm  window 
and  demonstrated  a  technology 
for  brazing  the  diamond  and 
heater  assembly  to  a  mallory 
metal  mount.  Pressure  tests  to 
2500  psi  showed  no  leak  or 
structural  problems  at  that  stage 
of  window  seal  development. 


the  diaphragm,  and  modifications  had  to  be 
made  to  the  ram  and  valve  body. 

Mission  Operations 

Pioneer  Venus  mission  controllers  had  to 
operate  two  different  spacecraft  at  the  same 
time.  Keep  in  mind  that  all  Pioneers  are  rela- 
tively unautomated  spacecraft,  designed  to 
minimize  costs.  This  required  mission  opera- 
tions to  maintain  24-hours-a-day  control  with 
careful  analysis  and  planning  at  short  notice. 
Although  ground-controlled  spacecraft  have 
flexibility  to  change  plans  and  objectives 
during  a  mission,  they  require  constant  moni- 
toring and  control.  Pioneer  Venus  control  and 
spacecraft  operations  were  at  the  Pioneer 
Mission  Operations  Center  (PMOC) 
(Figure  2-12)  at  Ames  Research  Center.  Bendix 
Field  Engineering  Corporation  provided 
support  services. 

Continued  operation  of  previously  launched 
Pioneer  spacecraft  made  activities  at  the 
Mission  Operations  Center  somewhat  more 
complicated.  Pioneers  6,  7,  8,  and  9  continued 
to  circle  the  Sun  and  to  return  interplanetary 
data.  Pioneer  10,  which  flew  past  Jupiter  in 
1973,  was  heading  out  of  the  Solar  System.  In 
its  travels,  it  continued  to  transmit  important 
information  from  previously  unexplored 
regions  of  space.  Pioneer  11,  which  flew  by 
Jupiter  in  1974,  was  on  its  way  to  the  first 
rendezvous  of  a  spacecraft  with  Saturn. 

All  command  information  originated  from 
the  PMOC.  The  Center  received  telemetry 
data  required  for  control  of  the  mission  and 
displayed  the  information  as  needed.  Com- 
puters allowed  personnel  to  enter  commands 
and  rapidly  interpret  the  spacecraft's  data 
stream  for  flight  controllers.  The  integrated 
team  working  at  the  Center  was  made  up  of 
dedicated  individuals  from  NASA  and  its 
support  contractor,  Bendix. 

Because  two  spacecraft  with  separate  missions 
were  involved,  two  flight  operations  groups 
were  on  hand:  an  Orbiter  group  and  a  Multi- 
probe  group.  Both  groups  had  a  science- 
analysis  team  that  determined  each  instru- 
ment's status  and  formulated  command 
sequences  for  that  mission.  They  also  each  had 
a  spacecraft  performance  analysis  team.  These 
teams  analyzed  and  evaluated  spacecraft 
performance  and  predicted  how  it  would 
respond  to  commands.  A  third  group  served 
both  spacecraft.  This  was  the  navigation  and 
maneuvers  group  that  took  care  of  spacecraft 
navigation,  orbital  injection  and  trim,  and 
probe  targeting. 

To  determine  spacecraft  trajectories,  JPL 
provided  computer  analysis  of  the  tracking 
information  from  the  Deep  Space  Network 
(DSN).  Support  groups  at  Ames  Research 

Center  and  at 
other  NASA 
facilities  also 
assisted  the 
mission  operations 
team.  They  helped 
with  computer 
software  develop- 
ment, mission 
control,  and 
off-line  data 

Data  Return,  Command 
and  Tracking 

To  track  all  six  spacecraft— four  probes,  Bus, 
and  Orbiter— NASA  used  the  DSN's  global 
system  of  large  parabolic  dish  antennas.  The 
large  antennas  at  each  site  were  essential  for 
critical  phases  of  the  mission.  These  events 
included  reorientation  of  the  spacecraft, 

velocity  corrections,  orbit  insertion,  and  entry 
of  the  four  probes  into  Venus'  atmosphere. 
The  large  antennas  also  were  involved  in 
special  science  events  such  as  radio-occultation 

The  DSN,  which  JPL  managed,  had  facilities 
located  at  approximately  120°  intervals  around 
Earth  (Figure  2-13).  As  the  Orbiter  and  the 
Multiprobe  appeared  to  set  at  one  station  due 

Figure  2-12.  The  Pioneer  Mission 
Operations  Center  (PMOC)  at 
NASA  Ames  Research  Center, 
California,  commanded  and 
controlled  all  spacecraft. 


Deep  Space 



Madrid  DSN 


Santiago  Spacecraft 

Tracking  and  Data 

Network  (STDN) 


Figure  2- 1 3.  NASA  used  the 
worldwide  system  of  the  Deep 
Space  Network  (DSN)  to 
communicate  with  the  space- 
craft during  the  mission,  to 
issue  commands  to  some  of 
them,  and  to  receive  scientific 
data  telemetered  from  all 
the  spacecraft. 


Figure  2-14.  (Top)  Two  of  these 
big  antennas  at  Goldstone, 
California,  and  Canberra, 
Australia,  maintained  contact 
with  the  probes  during  their 
penetration  of  Venus ' 
atmosphere.  NASA  used  the  two 
antennas  at  the  same  time  to 
ensure  that  none  of  the  data 
was  missed  during  this  one-hour 
descent.  (Bottom)  During  probe 
and  bus  penetration  of  Venus' 
atmosphere,  the  Deep  Space 
Network  handled  six  spacecraft 
at  once.  All  the  spacecraft 
transmitted  their  information 
directly  to  Earth,  as  this 
diagram  shows. 



to  the  rotation  of  the  Earth,  they  were  rising 
at  the  next  station.  The  DSN  had  six  26-m 
(85-ft)  antennas.  Two  were  at  Goldstone,  in 
California's  Mojave  Desert,  two  at  Madrid, 
Spain,  and  two  at  Canberra,  Australia.  (Officials 
later  upgraded  one  at  each  location  to  34  m,  or 
112  ft,  and  shut  down  the  remaining  26-m 
antennas  during  1981  budget  cuts.)  There  also 
were  three  64-m  (210-ft)  antennas  (Figure  2-14 
top),  one  each  at  the  three  locations.  During 
the  Pioneer  Venus  extended  mission,  officials 
upgraded  these  to  70  m  (230  ft).  In  the  critical 
2-hour  period  when  the  Bus  entered  the 

atmosphere  and  the  four  probes  descended  to 
the  surface,  scientists  relied  on  the  64-m 
(210-ft)  antennas  at  Goldstone  and  Canberra. 
They  used  these  antennas  to  receive  and  record 
data  from  all  five  spacecraft  at  the  same  time 
(Figure  2-14).  Two  additional  tracking  stations 
provided  special  data  gathering  for  the  probes' 
Differential  Long  Baseline  Interferometry 
(DLBI)  experiment.  These  were  the  9-m 
antenna  stations  that  were  part  of  the  Space- 
flight  Tracking  and  Data  Network  (STDN)  at 
Santiago,  Chile,  and  on  Guam. 

During  launch,  the  DSN,  with  the  help  of 
other  facilities,  tracked  each  spacecraft.  These 
other  facilities  were  the  Air  Force  Eastern  Test 
Range  tracking  antennas  and  elements  of 
NASA's  Spacecraft  Tracking  Data  Network. 
Four  instrumented  aircraft  from  Wright 
Patterson  Air  Force  Base  also  provided  tracking 

The  Deep  Space  Network  Stations  formatted 
incoming  telemetry.  From  there,  it  traveled 
over  the  high-speed  circuits  of  the  NASA 
Communications  System  (NASCOM)  to  the 
Pioneer  Mission  Computing  Center  (PMCC). 
There  computers  processed  it  to  supply  various 
types  of  real-time  display  information  for  all 
spacecraft  and  their  experiments.  The  comput- 
ers checked  for  unexpected  or  critical  changes 
in  data.  They  also  provided  information  to 
specialists  experienced  in  all  details  of  the 
spacecraft,  experiments,  and  the  ground 
system.  Their  analyses  ensured  that  space- 
craft were  always  controlled  correctly  for  the 
best  science  results.  Computers  at  Ames 
Research  Center  verified  outgoing  commands. 
They  then  sent  these  commands  to  the  Deep 
Space  Network  Stations  where  computers 
again  verified  the  commands  before  relaying 
them  to  the  spacecraft.  JPL  furnished  naviga- 
tion data  and  trajectory  computations  for  the 
Pioneer  spacecraft. 

Mission  specialists  made  several  changes  to  the 
DSN  for  its  use  in  the  Pioneer  Venus  mission. 
They  added  receivers  to  handle  five  different 
data  streams  at  the  same  time.  To  cope  with 
large  frequency  drifts,  they  installed  special 
wide  band  recorders.  Two  events  caused  the 
drifts:  (1)  shifts  in  probe  velocity  as  the  probes 
entered  Venus'  atmosphere  and 
(2)  atmospheric  effects  on  signal  propagation 
as  probes  descended  through  the  dense,  hot 
atmosphere.  To  make  sure  that  no  data  were 

lost  as  the  probes  plunged  through  the 
atmosphere,  the  DSN  took  special  precautions. 
They  provided  special  equipment  to  tune 
receivers  to  each  probe's  signals  and  to  record 
data  in  unsynchronized  form  for  special 
off-line  processing. 

The  PMCC  did  more  than  provide  telemetry 
for  mission  operations  and  quick  looks  at 
scientific  data.  The  Center  also  processed  all 
telemetry  to  supply  experiment  data  records  to 
principal  investigators  for  distribution  to  their 
team  members. 

Countdown  to  Launches 

During  February  1978,  pre-shipment  reviews 
took  place  at  the  Hughes  Aircraft  Company 
plant  in  El  Segundo,  California.  Following 
these  reviews,  NASA  shipped  the  spacecraft  to 
the  launch  site  at  Kennedy  Space  Center, 
Florida.  The  main  body  of  the  Orbiter  and  the 
high-gain  antenna  were  shipped  separately. 
When  they  arrived  in  Florida,  the  first  task  was 
to  mate  the  antenna  and  the  spacecraft.  Later, 
in  the  checkout  area,  engineers  tested  the 
complete  Orbiter  extensively  to  make  certain 
that  all  subsystems  and  scientific  instruments 
were  operating  correctly. 

After  these  tests  were  complete,  engineers 
installed  class  B  ordnance  (ordnance  that 
would  not  harm  the  spacecraft  or  test  person- 
nel if  it  were  fired  by  mistake).  The  spacecraft 
was  then  moved  to  Building  SAFE-2  where  staff 
loaded  the  rest  of  the  ordnance  and  32  kg 
(70  Ib)  of  hydrazine  propellant.  Hydrazine  was 
the  fuel  used  for  trajectory  corrections  and 
orientation  maneuvers.  Mission  personnel 
then  mated  the  spacecraft  with  an  adapter  that 
attached  it  to  the  launch  vehicle.  Following 
this  procedure,  they  transferred  the  spacecraft 
to  launch  pad  36  where  they  mated  it  to  the 
waiting  Atlas-Centaur. 


Figure  2-15.  NASA  successfully 
launched  the  first  Pioneer 
Venus  spacecraft,  the  Orbiter, 
on  20  May  1 978.  The  launch 
took  place  from  NASA  Kennedy 
Space  Center,  Florida. 


Once  the  spacecraft  was  on  the  launch  vehicle, 
another  series  of  tests  verified  that  the  space- 
craft and  its  systems  had  not  degraded  in  any 
way.  Then  followed  a  series  of  radio  frequency 
interference  (RFI)  tests.  These  tests  verified  that 
the  radar  for  tracking  the  vehicle  during  launch 
would  not  create  problems.  Specifically,  they 
ensured  the  radar  neither  interfered  with  the 
spacecraft  nor  affected  the  data  coming  from  it. 

After  several  practice  countdowns,  a  final  test 
with  the  DSN  determined  if  signals  from  the 

spacecraft  were  correct.  After 
10  days  of  various  tests, 
mission  officials  gave  the 
"go"  for  launch,  and  the 
countdown  began. 

There  were  no  holds.  The 
big  Atlas-Centaur  lifted  the 
Orbiter  into  the  Florida  skies 
on  its  way  to  Venus 
(Figure  2-15).  The  launch 
was  precisely  on  schedule — 
May  20,  1978,  at  1313  UT. 

Meanwhile,  during  April,  the 
Multiprobe  completed  its 
pre-shipment  review  at 
Hughes.  The  Large  Probe  was 
shipped  separately  from  the 
Bus  and  the  three  Small 
Probes.  As  soon  as  the 
spacecraft  arrived  at  the 
Kennedy  Space  Center 
checkout  area,  the  Small 
Probes  were  removed  from 
the  Bus.  Each  was  thoroughly 
checked,  as  was  the  Large 
Probe.  During  checkout,  the 
flight  batteries  remained 
under  strict  thermal  control. 
They  could  not  be  on  board 
the  probe  for  testing  because  engineers  could 
not  put  them  through  charge/discharge  cycles. 
Other  batteries  were  used  for  the  tests,  and  the 
flight  batteries  were  the  last  items  installed  on 
the  probe  at  the  end  of  checkout. 

Tests  in  the  thermal  vacuum  chamber  at 
Martin  Orlando  verified  pressure  vessel  seals. 
The  gas  within  each  probe  contained  a  trace  of 
helium.  For  24  hours,  test  engineers  sampled 
the  vacuum  chamber  contents.  Helium  in  the 
vacuum  chamber  samples  would  have  revealed 

a  seal  failure.  None  appeared 
for  any  of  the  separately 
tested  probes.  All  passed  the 
leakage  test  satisfactorily. 

Back  at  the  Kennedy  Space 
Center,  technicians  placed 
the  probes  on  the  Bus.  Next, 
they  installed  pyrotechnics — 
explosive  bolts  for  the  Large 
Probe  and  bolt  cutters  for 
each  Small  Probe.  These 
devices  would  release  the 
probes  from  the  Bus.  Also, 
hydrazine  was  loaded  into 
the  Bus. 

The  Multiprobe  was  then 
moved  to  the  launch  pad, 
mated  with  the  Atlas-Centaur, 
and  underwent  a  final 
checkout.  Engineers  could 
only  make  a  very  brief  check 
of  the  radio  frequency  link  to 
each  probe.  The  probes  were 
all  warm,  near  the  ambient 
Florida  temperature.  However,  when  they 
reached  the  atmosphere  of  Venus,  they  would 
be  very  cold.  To  avoid  exceeding  temperature 
limits  for  equipment  within  the  heavily 
insulated  probes,  tests  had  to  be  extremely 
brief.  Personnel  turned  on  each  probe's  radio 
frequency  transmitters  for  a  very  short  time  to 
verify  their  signals.  For  the  Large  Probe, 
because  the  antenna  was  in  a  support  cone, 
engineers  had  to  connect  a  pickup  antenna  to 
an  outside  antenna.  Without  this,  the  probe 
could  not  relay  its  signal  to  the  DSN  for 
the  test. 

Figure  2- 16.  On  8  August  7  978, 
slightly  less  than  three  months 
after  the  Orbiter  left  Earth,  NASA 
launched  the  second  spacecraft, 
the  Multiprobe,  from  the 
Kennedy  Space  Center,  Florida. 
It  followed  the  Orbiter  to  a 
rendezvous  with  Venus  in 
December  of  that  year. 

loaded  the  liquid  helium  into  the  Centaur, 
they  discovered  that  the  helium  truck  carried 
less  liquefied  gas  than  it  should  have.  As  a 
result,  the  countdown  went  on  hold.  It 
resumed  on  August  7  at  1830  EOT.  NASA 
finally  sent  the  Multiprobe  on  its  way  to 
Venus  at  0733  UT  on  August  8,  1978 
(Figure  2-16). 

The  first  U.S.  mission  into  the  cloud-shrouded 
atmosphere  of  Earth's  mysterious  sister  planet 
was  successfully  on  its  way. 


Finally,  the  Multiprobe  countdown  began.  All 
went  well  until  close  to  the  scheduled  launch 
date  of  August  6,  1978.  Then,  as  technicians 


'     '        - 


A  new  era  dawned  with  an  announcement  from 
the  Commander,  Air  Force  Missile  Test  Center, 
Cape  Canaveral,  Florida,  October  11,  1958: 
"The  United  States  launched  a  three-stage  exper- 
imental space  vehicle  at  the  Atlantic  Missile 
Range  at  Cape  Canaveral,  Florida,  at  0342  EST 
this  morning.  The  launching  was  accomplished 
by  the  Air  Force  under  the  direction  of  the 
National  Aeronautics  and  Space  Administration 
(NASA).  It  was  the  second  flight  test  of  a  num- 
ber of  small  unmanned  space  vehicles  designed 
to  gather  scientific  data  as  a  part  of  the  U.S. 
International  Geophysical  Year  program 
which  is  sponsored  by  the  National  Acad- 
emy of  Sciences  with  the  support  of  the 
National  Science  Foundation.  The  vehicle 
is  composed  of  the  Thor  intermediate 
range  ballistic  missile  as  the  first  stage 
(or  booster),  a  modified  Vanguard 
second  stage,  and  an  advanced 
version  of  the  Vanguard  third  stage. 
Topping  this  vehicle  is  a  highly 
instrumented  scientific  payload." 

A  short  while  later,  another 
announcement  followed: 
"The  Department  of  Defense 
gave  the  name  'Pioneer' 
today  to  the  payload  of 
the  successfully  launched 
U.S.  lunar  probe  rocket, 
the  first  man-made 
object  known  to  escape 
the  Earth's  gravitational  field." 

This  then  was  the  genesis  of  an  interplanetary 
spacecraft  series  bearing  the  name  Pioneer. 
Their  missions  were  many:  explore  beyond 
Earth  and  be  first  to  visit  Jupiter  and  Saturn, 
probe  into  the  Solar  System's  outermost 
reaches,  and  penetrate  the  atmosphere  of 
mysterious  cloud-shrouded  Venus  and  observe 
that  planet  from  orbit  for  14  years. 

Several  studies  in  the  years  after  the  first  Air 
Force  lunar  probes  showed  how  unmanned 
spacecraft  might  explore  the  Solar  System.  In 
early  1960,  NASA  transferred  the  solar  probe 
study  program  to  Ames  Research  Center.  There 
it  continued  under  the  leadership  of  Charles  F. 
Hall  and  a  team  appointed  September  14  by 
Smith  J.  DeFrance,  Director  of  the  Center. 
Other  members  of  the  team  were  J.  Dimeff, 
C.  F.  Hansen,  W.  A.  Mersman,  R.  T.  Jones, 
H.  F.  Matthews,  H.  Hornby,  W.  J.  Kerwin,  and 
C.  A.  Hermach.  In  these  startup  years,  scien- 
tists envisioned  a  spacecraft  approaching 
within  44,850,000  km  (27,870,000  miles)  of 
the  Sun. 

In  succeeding  years,  Hall  sought  support  from 
NASA  Headquarters  for  this  idea.  He  won 
approval  from  Edgar  M.  Cortright,  then 
Deputy  Director  of  the  Office  of  Space  Science, 
to  develop  an  interplanetary  Pioneer  as  a  step 
toward  a  solar  probe.  Ames  management  con- 
curred and,  in  April  1962,  Space  Technology 
Laboratories  of  Redondo  Beach,  California,  com- 
pleted a  feasibility  study.  This  study  developed 
a  concept  for  a  spin-stabilized  spacecraft  with 
special  features.  Specifically,  it  would  meet  design 
constraints  of  low  weight,  low  cost,  and  quick 
design  and  fabrication  for  various  missions  to 
explore  interplanetary  space  and  its  environment. 

Contracts  were  awarded  following  competitive 
bidding,  and  project  personnel  planned  the 

With  the  launch  of  Pioneer  1 
on  October  11,  1958,  the 
Pioneer  series  of  spacecraft 
began.  The  first  part  of  this 
chapter  gives  a  history  of  the 
series,  leading  up  to  Pioneer 
Venus.  A  detailed  review  of 
the  interplanetary  mission's 
two  main  components — the 
Orbiter  and  Multiprobe — 
appears  in  the  remaining 
chapter  sections.  In  addition 
to  the  spacecraft's  structural 
details,  the  text  also  covers 
these  features:  data  han- 
dling, commands,  antennas, 
power  sources,  and  com- 
munications from  Earth. 


Figure  3-1.  The  Pioneer  space- 
craft series  started  with  the 
Interplanetary  Pioneer  6, 
launched  in  7  965.  (The  Air  Force 
had  originally  used  the  name  for 
an  earlier  lunar  probe  series.) 
The  Pioneer  missions  culminated 
in  the  Pioneer  Venus  spacecraft, 
launched  in  1 978. 


first  launch  for  1965.  The  Pioneer  program 
originally  consisted  of  five  spacecraft  and  their 
experiments.  Ames  Research  Center  managed 
the  project,  TRW  Systems  built  the  spacecraft, 
and  experimenters  provided  the  scientific 
instruments.  Engineers  designed  all  these 
spacecraft  to  orbit  the  Sun  in  approximately 
the  plane  of  Earth's  orbit  (the  ecliptic  plane), 
some  initially  directed  inside  Earth's  orbit, 
some  outside. 

The  first  of  the  Pioneer  series  spacecraft 
launched  by  NASA  was  Pioneer  6  (Figure  3-1) 
on  December  15,  1965.  On  August  17  of  the 
following  year,  Pioneer  7  was  launched  suc- 
cessfully. Pioneer  8  followed  on  December  13, 
1967,  and  Pioneer  9  on  November  8,  1968. 
The  final  launch  in  the  series  was  on 
August  27,  1969.  After  214  seconds,  however, 
the  Delta  booster  on  this  flight  developed 
problems.  The  Range  Safety  Officer  ordered  the 
booster  destroyed  484  seconds  into  the  flight. 
The  spacecraft  was  lost. 

These  Pioneer  spacecraft  showed  the  practical- 
ity of  spin-stabilizing  the  spacecraft  to  steady  it 
and  simplify  control  of  its  orientation.  The 
spacecraft  also  proved  reliable  and  operable  far 
beyond  their  initial  missions.  The  scientific 
results  were  impressive.  Pioneer  missions  con- 
firmed that  there  was  a  spiral  solar  magnetic 
field  imbedded  in  the  plasma  that  streams  out- 
ward from  the  Sun.  They  also  confirmed  the 
structure  of  Earth's  bow  shock  and  of  the  mag- 
netopause.  They  mapped  a  geomagnetic  tail 

and  provided  insights  into  what  happens  in 
interplanetary  space  when  a  solar  flare  erupts. 
The  missions  recorded  energy  spectra  of  solar 
electrons  and  positive  ions  and  showed  the 
solar  wind's  average  electron  temperature  to  be 
about  100,000  K  during  times  of  low  solar 
activity.  Cosmic  ray  telescopes  showed  that, 
during  solar  minimum,  most  high  energy  cos- 
mic ray  particles  originated  outside  the  Solar 
System.  However,  even  at  solar  minimum,  the 
telescopes  showed  low  energy  cosmic  rays 
were  mainly  of  solar  origin.  The  spacecraft  also 
measured  shapes  of  plasma  clouds  and  the 
electric  fields  in  interplanetary  space. 

An  important  Pioneer  discovery  was  that 
cosmic  dust  is  not  a  serious  hazard  to  man  or 
spacecraft  operating  outside  Earth's  atmo- 
sphere. Also,  astronomers  improved  Solar 
System  constants  and  ephemerides  (tables 
giving  a  celestial  body's  coordinates  at  a 
number  of  specific  times  during  a  given 
period).  They  accomplished  this  by  accurately 
tracking  Pioneer  spacecraft  in  their  heliocen- 
tric orbits.  The  precision  of  other  astronomical 
measurements  also  improved.  These  included 
gravitational  constants  for  Earth  and  the 
Moon,  the  mass  ratio  of  Earth  and  the  Moon, 
and  the  distance  of  Earth  from  the  Sun  (the 
astronomical  unit). 

In  1969,  engineers  designed  a  new  class  of 
Pioneer  spacecraft:  a  low-cost,  lightweight, 
spin-stabilized  spacecraft  for  flybys  of  other 
planets.  The  first  two  Pioneers  of  this  class 
were  Pioneers  10  and  11,  originally  designed  to 
fly  by  Jupiter.  These  spacecraft  were  highly 
successful  in  withstanding  the  intense  radia- 
tion as  they  passed  through  the  radiation  belts 
of  Jupiter  in  1973  and  1974.  They  also  suc- 
ceeded in  maintaining  contact  with  Earth 
from  the  enormous  distances  of  the  outer  Solar 
System.  As  a  result,  planners  added  a  new  task 
to  Pioneer  11  's  mission.  The  craft  now  would 

Table  3-1.  The  Pioneer  Explorers 


Launch  date 



Pioneer  1 
Pioneer  2 
Pioneer  3 
Pioneer  4 

11  Oct  1  958 
8  Nov1958 
6  Dec  1  958 
3  Mar  1959 


Reached  72,765  miles  from  Earth 
Reached  963  miles  only 
Reached  63,580  miles  only 
Passed  37,300  miles  from  Moon  and  entered 
solar  orbit 

Pioneer  5 
Pioneer  6 
Pioneer  7 
Pioneer  8 
Pioneer  9 
Pioneer  E 

11  Mar  1960 
16  Dec  1965 
1  7  Aug  1  966 
13  Dec  1967 
8  Nov  1  968 
7  Aug  1969 

Solar  orbit 
Solar  orbit 
Solar  orbit 
Solar  orbit 
Solar  orbit 
Solar  orbit 

Entered  solar  orbit 
Still  operating 
Still  operating 
Still  operating 
Signal  lost  in  1983 
Launch  failure 

Pioneer  10 
Pioneer  1  1 

2  Mar  1972 
5  Apr  1973 


Flew  by  Jupiter  in  1  973;  still  operating  in  outer 
solar  system 
Flew  by  Jupiter  in  1  974;  flew  by  Saturn  in  1  979; 
still  operating  in  outer  solar  system 

Pioneer  12 
Pioneer  1  3 

20  May  1978 
8  Aug  1978 


Entered  orbit  around  Venus  on  Dec  4,  1978. 
Orbited  successfully  until  Oct  8,  1  992 
Reached  Venus  Dec  9,  1979  and  all  probes  and 
bus  entered  the  atmosphere  successfully 

speed  across  the  Solar  System  high  above  the 
ecliptic  plane.  Then,  in  1979,  it  would  fly  by 
Saturn  before  following  Pioneer  10  into 
interplanetary  space  in  the  outer  Solar  System, 
but  in  the  opposite  direction.  Pioneer  11 
proceeded  ahead  of  the  Sun  (called  the  Solar 
Apex  direction),  and  Pioneer  10  traveled  in  the 
Anti-Solar  Apex  direction. 

Pioneers  10  and  11  showed  that  spacecraft 
could  safely  pass  through  the  asteroid  belt  and 
through  the  Jovian  radiation  belts.  They  also 
made  significant  discoveries  about  the  Solar 
System's  two  largest  planets.  They  found  that 
Jupiter  must  be  a  liquid  planet  and  that  its 
atmosphere  is  heated  uniformly  from  equator 
to  pole  and  in  day  and  night  hemispheres. 
They  revealed  that  Jupiter's  magnetosphere  is  a 
pulsating  volume  of  particles  and  fields  that 
are  stirred  up  by  its  inner  satellites.  The 
spacecraft  found  three  distinct  regions  and 
showed  that  the  planet  is  the  source  of 
energetic  particles  hurtled  across  the  Solar 

System.  After  they  had  confirmed  the  intensity 
and  orientation  of  Jupiter's  magnetic  field, 
Pioneer  11  detected  Saturn's  magnetic  field. 

The  spacecraft  imaged  Jupiter's  polar  regions 
for  the  first  time,  and  Pioneer  1 1  observed 
Saturn's  rings  from  the  shadowed  side. 
Pioneer  1 1  uncovered  several  new  features  in 
the  Saturn  ring  system,  including  a  thin  A-ring 
beyond  the  F-ring.  It  also  discovered  addi- 
tional satellites  of  Saturn.  The  spacecraft  mea- 
sured magnetic  field  strengths,  and  Pioneer  10 
was  the  first  to  obtain  images  of  the  Galilean 
satellites  and  Pioneer  11  the  first  of  Titan. 

Pioneers  10  and  11  were,  without  question, 
highly  successful  precursors  to  the  Voyager  1 
and  2  outer-planet  spacecraft  that  the  Jet 
Propulsion  Laboratory  developed  and 
launched  in  1977. 

The  Pioneer  Venus  spacecraft — the  Orbiter  and 
the  Multiprobe — were  the  next  steps  in  the 


Figure  3-2.  The  main  body  of 
the  Pioneer  Venus  spacecraft,  a 
simple  cylinder,  was  the  common 
design  for  both  Orbiter  and 
Multiprobe.  It  had  shelves  for 
equipment,  thrusters  for 
maneuvering,  an  omni  antenna, 
and,  for  the  Orbiter  only,  a 
solid-propellant,  orbit-insertion 
rocket  motor. 

Solar  array 

Radial  thruster 

Forward  axial 

Aft  omni  antenna 

Orbit  insertion  motor 
(Orbiter  only) 

Equipment  shelf 


evolution  of  this  highly  successful  line  of  trail- 
blazing  interplanetary  Pioneer  probes.  One  of 
the  Venus  spacecraft  became  a  true  planetary 
probe  when  it  carried  several  spacecraft  into 
the  Venusian  atmosphere,  rather  than  just 
flying  by  or  orbiting  the  planet.  Whereas  TRW 
Systems  built  the  previous  Pioneers,  Hughes 
Aircraft  Company  built  the  Pioneer  Venus  space- 
craft. Ames  Research  Center  continued  in  the 
project  management  role. 

The  Pioneer  exploring  spacecraft  appear  in 
Table  3-1.  Information  includes  spacecraft 
names,  launch  dates,  intended  missions,  and 
a  summary  of  their  status. 

The  Orbiter 

The  Orbiter  was  the  spin-stabilized  platform 
for  the  orbital  mission's  12  scientific  instru- 

ments. To  reduce  the  mission's  cost,  it  used 
the  basic  Pioneer  Bus,  common  to  both  the 
Orbiter  and  the  Multiprobe. 

The  main  body  of  the  spacecraft  (Figure  3-2) 
was  a  flat  cylinder  2.5  m  (8.2  ft)  in  diameter 
and  1.2  m  (4  ft)  high.  In  the  upper  or  forward 
end  of  the  cylinder  was  a  circular  equipment 
shelf  with  an  area  of  4.37  m2  (47  ft2).  All  the 
spacecraft's  scientific  instruments  and  elec- 
tronic subsystems  were  on  this  shelf  (see 
Chapter  4  for  descriptions  of  these  instru- 
ments). Engineers  fastened  the  shelf  on  the 
forward  end  of  a  thrust  tube  that  connected 
the  spacecraft  to  the  launch  vehicle.  Twelve 
equally  spaced  struts  supported  the  periphery 
of  the  shelf  from  the  lower  part  of  the  thrust 
tube.  Below  the  shelf,  15  thermal  louvers 
(Figure  3-3)  controlled  heat  radiation  from  an 

Figure  3-3.  Thermal  louvers 
controlled  the  internal  tem- 
perature of  the  spacecraft. 
These  flight  model  louvers 
appear  partially  open.  Cold 
strips  of  Kapton  film  were 
added  to  the  inboard  side  of 
the  blades  to  radiate  energy 
to  the  spacecraft.  This  reduced 
blade  temperature  as  required. 

equipment  compartment  that  was  between  the 
shelf  and  the  top  of  the  spacecraft.  A  cylindri- 
cal solar  array  (Figure  3-4),  attached  to  the 
shelf  by  24  brackets,  formed  the  circumference 
of  the  flat  cylinder  of  the  spacecraft. 

On  top  of  the  spacecraft  was  a  1.09-m  (3.6-ft) 
diameter,  despun,  high-gain,  parabolic  dish 
antenna  (Figure  3-5).  The  antenna  was  on  a 
mast  so  that  its  line  of  sight  cleared  equipment 
mounted  outside  the  spacecraft.  The  despun 
design  allowed  the  antenna  to  be  mechanically 
directed  to  continuously  face  Earth  from  the 
spinning  spacecraft.  The  antenna  operated  at 
S-  and  X-bands. 

The  spacecraft  also  carried  a  solid-propellant 
rocket  motor  (Figure  3-6)  with  18,000  N 
(4046  Ib)  of  thrust.  This  thrust  would  deceler- 
ate the  spacecraft  by  3816  km/hr  (2371  mph) 
and  place  it  into  an  orbit  around  Venus. 
Including  the  antenna  mast,  the  Orbiter  was 
almost  4.5  m  (15  ft)  high,  and  it  weighed 
553  kg  (1219  Ib)  on  Earth.  Its  launch  weight 
included  45  kg  (100  Ib)  of  scientific  instru- 
ments and  179  kg  (395  Ib)  of  rocket  propellant. 

A  maneuvering  system  for  the  Orbiter's  basic 
Bus  controlled  its  rate  of  spin  and  made  course 
and  orbit  corrections.  The  system  also  main- 
tained the  spin  axis  orientation,  which  was 
usually  perpendicular  to  the  ecliptic  plane. 
Beneath  the  equipment  compartment  and 

attached  to  the  thrust  tube  were  two  conical 
hemispheric  propellant  tanks  (Figure  3-7). 
Each  tank  was  32.5  cm  (12.8  in.)  in  diameter. 
Initially,  these  tanks  stored  32  kg  (70  Ib)  of 
hydrazine.  This  hydrazine  was  the  propellant 
for  three  axial  and  four  radial  thrusters 
(Figure  3-8).  These  thrusters  changed  the 
attitude,  velocity,  or  orbital  period  and  spin 
rate  of  the  spacecraft  during  the  mission.  Two 
axial  thrusters  aligned  with  the  axis  of  spin 
and  were  at  the  top  and  bottom  of  the  Bus 
cylinder.  They  were  diagonally  opposite  each 
other  and  pointed  in  opposite  directions. 
When  ground  controllers  had  to  turn  the  spin 

Figure  3-4.  A  cylindrical 
solar  array  formed  the 
circumference  of  the  Bus 
cylinder  and  provided 
electrical  energy  for  the 
spacecraft  and  its  payload 
of  scientific  instruments. 


Figure  3-5.  A  high-gain  parabolic 
dish  antenna,  mounted  on  a 
mast,  could  be  despun  relative  to 
the  spacecraft  so  that  it  could 
point  toward  Earth. 

Figure  3-6.  A  solid-propellant 
rocket  motor  provided  thrust 
to  place  the  Orbiter  into  an 
elliptical  orbit  around  Venus. 
Thiokol  Corporation  supplied 
this  orbit  insertion  motor. 
The  picture  shows  the  motor 
case  and  the  nozzle  closure 
of  titanium  alloy  forgings. 
The  case  consisted  of  two 
0. 028-inch  thick  hemispheres 
joined  by  a  single  weld.  The 
nozzle  closure,  which  fastened 
to  the  case,  was  the  structural 
element  of  the  nozzle.  Integral 
bosses  allowed  attachment 


axis,  they  fired  the  thrusters  in  pulses  in 
opposite  directions.  To  speed  up  or  slow  down 
the  spacecraft  along  the  direction  of  its  spin 
axis,  they  fired  only  one  thruster  in  pulses. 
They  did  this  at  two  points  180°  apart  in  the 
spacecraft's  rotation,  and  chose  the  top  or 
bottom  thruster,  depending  on  the  direction 
required  for  the  velocity  change. 

A  third  thruster 
unit  was  at  the 
bottom  of  the 
thrust  cylinder 
and  permitted 
continuous  firing 
of  two  bottom 
thrusters.  This 
firing  allowed 
moves  in  an  axial 
direction  so  mis- 
sion controllers 
could  change  the 
spacecraft's  orbit. 

The  four  radial 
thrusters  were  in 
two  pairs,  point- 
ing in  opposite  directions.  They  were  in  a 
plane  approximately  perpendicular  to  the  spin 
axis.  (This  plane  passed  through  the  center  of 
gravity  of  the  spacecraft.)  The  radial  thrusters 
were  used  to  change  the  spacecraft's  velocity 
in  a  direction  perpendicular  to  the  spin  axis. 
The  thrusters  also  controlled  the  spin  rate. 
They  were  equally  positioned  on  the  Bus 
cylinder's  periphery.  Firing  two  of  them  180° 

apart  slowed  the 
spin  rate.  Firing 
the  other  two 
increased  the 
spin  rate. 

Sun  sensors  and 
a  shelf-mounted 
star  sensor  pro- 
vided attitude 
references  to  con- 
trol the  spacecraft. 
Each  instrument 
had  a  slit  aperture 
for  its  field  of  view. 

The  Orbiter's 

features  consisted  of  six  basic  assemblies. 
These  were  the  despun  antenna  assembly,  the 
bearing  and  power  transfer  assembly  and  its 
support  structure,  the  equipment  shelf,  the 
solar  array,  the  orbit  insertion  motor  and  its 
case,  and  the  thrust  tube  (Figure  3-9). 

On  Venus,  the 
intensity  of  the 
Sun's  radiation  is 
nearly  twice  that 
on  Earth.  Pioneer 
Venus'  thermal 
design  isolated 
equipment  from 
extremes  of  solar 
heat  during  the 
mission.  To  keep 
the  spacecraft's 
critical  elements  at 
the  right  tempera- 
ture, there  were 
electric  heaters 
that  ground  con- 
trol could  turn  on. 
The  solid  propel- 

lant  rocket,  which  inserted  the  spacecraft  into 
orbit,  and  the  safeing  and  arming  devices 
required  temperature  control  early  in  the 
mission.  Also,  throughout  the  mission,  the 
hydrazine  propellant  had  to  stay  unfrozen.  But 
what  would  happen  if  equipment  that  devel- 
oped heat  during  its  operation  was  off  for  too 
long?  Mission  designers  planned  for  this 

Figure  3-7.  Hydrazine  pro- 
pellant for  maneuvering 
thrusters  was  stored  in  two 
tanks  within  the  spacecraft. 
Each  tank,  made  from  titanium 
alloy,  held  16  kg  (35  Ib)  of 
hydrazine  under  pressure. 

Figure  3-8.  Small  thrusters 
controlled  the  spacecraft's 
orientation  and  spin  rate. 
They  were  in  two  redundant 
groups,  positioned  on  the 
spacecraft  so  that  ground 
control  could  change  the 
spacecraft's  velocity,  spin 
rate,  and  attitude.  This 
photograph  shows  a 
thruster  assembly. 


Figure  3-9.  Principal  elements 
of  the  Orbiter  spacecraft  appear 
on  these  cross-section  and 
side  views.  (See  Table  2-2, 
page  29,  for  instrument 


Magnetometer  boom 

Magnetometer  boom 

=  240° 

234.00  in. 
to  S/C  Z  axis 




Forward  axial 

90° +Y 




Forward  axial  thruster 


thermal  barrier 

and  blanket 


High  gain 



High  gain 

Star  sensor 

Radial  thruster 
cutouts  (4) 

•  Star  sensor 

Radial  thrusters  (4)  ref 
Aft  axial  thrusters  (2) 
Equipment  shelf 

Aft  omni  antenna 
Propellant  tank  (2) 

Orbit  insertion  motor 
Separation  plane 
S/C  C.C.  at  launch 
S/C  C.G.  after  VOI 

Note:  Star  sensor,  tanks,  axial  thrusters,  and  aft 
omni  antenna  shown  rotated  for  clarity 

situation  by  including  other  heaters  that  could 
raise  the  spacecraft's  internal  temperature. 

Data-Handling  Subsystem 

A  data-handling  subsystem  (Figure  3-10) 

within  the  Orbiter  conditioned  and  integrated 

all  analog  and  digital  telemetry  data  into  for- 
mats that  ground  control  selected  by  radio  com- 
mand. Resulting  information  went  to  the  com- 
munications subsystem  for  modulation  of  the 
downlink  (spacecraft-to-Earth)  S-band  carrier. 
Twelve  telemetry  storage,  playback,  and  real- 

Figure  3- 1 0.  This  block  diagram 
shows  the  basic  elements  of 
the  data  handling  subsystem 
common  to  the  Orbiter  and 
the  Multiprobe. 

time  data  rates  between  8  and  2048  bits/sec 
were  available.  During  interplanetary  cruise, 
the  Orbiter  used  a  rate  of  1024  bits/sec. 

The  data-handling  subsystem  included  a  data 
memory  with  two  data  storage  units.  Each  unit 
had  a  capacity  of  524,288  bits  (equivalent  to 
1024  minor  frames  of  telemetry).  The  sub- 
system was  primarily  for  use  during  an  Earth 
occupation  when  the  spacecraft  was  behind 
Venus  and  not  able  to  communicate  with 
Earth.  During  this  period,  which  could  last  up 
to  26  minutes,  the  data  memory  could  store 
just  over  1  million  bits  of  data.  That  translated 
into  an  average  maximum  rate  of  672  bits/sec. 

For  shorter  occultation  periods,  the  bit  rates 
could  be  higher.  Data  were  stored  or  read  at 
the  commanded  bit  rate.  If,  for  any  reason,  the 
Deep  Space  Network  could  not  receive  data 
from  the  spacecraft,  the  data  could  remain  in 
these  data  storage  units. 

The  Orbiter  data-handling  system  accepted 
information  from  spacecraft  subsystems  and 
the  scientific  experiments  in  several  forms. 
These  were  serial  digital,  analog,  and  one-bit 
bilevel  (on/of 0-  The  system  converted  analog 
and  one-bit  data  to  serial  digital  form  and 
arranged  all  information  in  formats  for  trans- 
mission to  Earth.  This  transmission  was  a 


Pioneer  Venus  Orbiter  format  assignments 

Figure  3-11.  Assignment  of  data 
formats  for  the  Orbiter  appear 
in  this  figure.  PER  refers  to  the 
periapsis  portion  of  the  orbit, 
APO  to  apoapsis.  PBK  is  for 
playback.  The  various  scientific 
instruments  appear  by  their 
project  acronyms  (see  Table  2-3). 



Telemetry  formats/Words  per  telemetry  frame 












Sub  D 



















































































































































































*Spare  bits 

continuous  sequence  of  major  telemetry 
frames,  each  with  64  minor  frames.  Each 
minor  frame,  in  turn,  contained  64  eight-bit 
words,  or  a  total  of  512  bits  per  minor  frame. 
The  words  of  a  minor  frame  came  in  1  of  13 
formats  that  ground  control  could  select  by 
radio  command.  Each  minor  frame  contained 
science  and  engineering  data  at  the  com- 
manded bit  rate,  subcommutated  data, 
spacecraft  ID  data,  and  frame  synchronization 
data.  Three  subcommutated  data  formats 
belonged  to  each  minor  frame.  One  was  for 
slowly  changing  science  and  science  house- 
keeping data,  and  the  other  two  were  for 
slowly  changing  spacecraft  engineering  data. 

The  Orbiter  had  a  total  of  14  telemetry 
formats.  Five  formats,  PERA,  PERB,  PERC, 
PERD,  and  PERE,  were  periapsis  formats.  These 
formats  allowed  mission  controllers  to  change 
the  emphasis  for  part  or  all  of  a  periapsis 
period.  Two  formats,  APOA  and  APOB,  were 
for  measurements  during  apoapsis  when  the 

spacecraft  was  relatively  distant  from  Venus. 
The  Launch-Cruise,  or  LACR,  format  furnished 
a  higher  rate  of  engineering  data.  At  the  same 
time,  LACR  permitted  measurements  by 
instruments  capable  of  interplanetary  observa- 
tions. The  Playback,  or  PBK,  and  the  Data 
Memory  Read  Out,  or  DMRO,  formats  permit- 
ted reading  out  data  from  the  Data  Storage 
Unit,  or  DSU.  However,  PBK  read  with 
realtime  scientific  data,  while  DMRO  excluded 
them.  The  Command  Memory  Read  Out,  or 
CMRO,  format  permitted  a  check  of  the 
command  memory  load.  There  also  was  an 
engineering  format  to  furnish  high  rate 
engineering  data  for  diagnostic  purposes.  To 
furnish  high  rate  data  from  the  attitude 
control  system,  there  was  an  attitude  control 
system  format,  or  ACS.  A  14th  format  could 
furnish  high-rate  data  of  a  few  selectable 
parameters  for  diagnostic  purposes.  The  nine 
formats  that  include  real-time  scientific  data 
appear  in  Figure  3-11. 


The  basic  command  system  accepted  a 
pulse-code-modulated,  frequency  shift-keyed, 
phase-modulated  (PCM/FSK/PM)  data  stream. 
The  stream  was  at  a  fixed  rate  of  4  bits/sec — the 
incoming  commands  from  Earth  via  the  radio 
receivers.  Each  command  word  consisted  of 
48  bits,  including  13  bits  for  synchronization. 
This  structure  resulted  in  a  one-in-a-million 
probability  of  the  spacecraft  accepting  a  false 
command.  The  system  had  a  total  of  192  pulse 
commands  and  12  magnitude  commands. 
Command  demodulators  activated  the  system, 
converted  the  signal  to  a  usable  binary  bit 
stream,  and  passed  it  to  cross-connected  com- 
mand processors.  The  spacecraft  routed  each 
command  it  received  to  the  addressed  destina- 
tion for  immediate  action  or  stored  it  for  later 
execution.  Both  command  memories  could 
store  up  to  128  commands  or  time  delays.  The 
command  subsystem  could  completely  decode 
each  assigned  command  and  generate  an  execu- 
tion command.  Or  it  could  partially  decode  the 
command,  which  was  completely  decoded  at  its 
destination.  Spacecraft  units  received  commands 
from  redundant  command  output  modules. 

Antenna  Systems 

The  Orbiter  carried  a  despun,  high-gain,  para- 
bolic antenna.  At  S-band,  this  antenna  directed 
a  7.6-degree  beam  toward  the  Earth  throughout 
the  mission.  The  antenna  dish  was  109  cm 
(43  in.)  in  diameter,  and  it  concentrated  the 
Orbiter's  signal  316  times  by  directing  it  into 
the  narrow  beam.  During  the  mission,  the 
distance  between  Earth  and  Venus  changed  by 
203  million  km  (126  million  miles).  Engineers 
designed  the  high-gain  antenna  to  return  data 
at  the  required  rates  over  the  greatest 
mission  distance. 

The  high-gain  antenna  dish,  a  sleeve  dipole 
antenna,  and  a  forward  omnidirectional 
antenna  were  all  on  a  mast  that  projected  2.9  m 

(9.8  ft)  along  the  s'pin  axis  from  the  top  of  the 
basic  cylinder  of  the  spacecraft  (Figure  3-12). 
The  sleeve  dipole  radiated  in  a  flat  pattern  in  a 
plane  perpendicular  to  the  spin  axis.  It  pro- 
vided backup  if  the  despin  mechanism  failed 
and  ground  control  could  not  point  the  dish 
antenna  toward  Earth. 

Both  omnidirectional  antennas — one  on 
the  antenna  mast  and  the  other  aft  of  the 
spacecraft — radiated  in  a  hemispherical  pattern. 
This  design  provided  low-gain  radiation  in 
all  directions  around  the  spacecraft.  At  any 
orientation,  the  spacecraft  could  receive  com- 
mands from  Earth  and  communicate  at  low 

One  of  two  electric  motors  despun  the  three 
antennas  on  the  mast  relative  to  the  spinning 
spacecraft.  The  mast  was  attached  to  a  bearing 
assembly  flange  that  was  on  the  Bus  thrust 
tube's  upper  end.  A  series  of  transfer  switches 
electrically  connected  the  three  antennas  to  the 
spinning  spacecraft's  transmitters.  These  con- 
nections ran  through  a  dual  frequency  rotary 
joint  (Figure  3-13).  Pulse  commands,  in  turn, 
controlled  the  switches.  The  commands  trav- 
eled through  slip  rings  and  brushes  on  the  bear- 
ing and  power-transfer  assembly  that  supported 
and  rotated  the  mast  relative  to  the  spacecraft. 

A  control  system  provided  redundant  electron- 
ics to  control  the  despin  mechanism.  The  sys- 
tem also  drove  either  one  of  the  two  electric 
motors.  Depending  on  signals  from  the  Sun 
and  star  sensors,  despin  control  electronics 
generated  motor  torque  commands.  The  para- 
bolic antenna  could  be  pointed  in  elevation  by 
a  motor-driven  jackscrew. 

The  Orbiter  carried  a  750-mW,  X-band  trans- 
mitter for  radio  experiments  during  occulta- 
tion.  The  signal  frequency  of  this  transmitter 
was  1-1/3  times  that  of  the  main  S-band 


Figure  3-12.  The  antenna  mast 
carried  several  antennas  and  the 
parabolic  dish. 


Omni  antenna 



Backup  high-gain 

Mechanically  despun 
antenna  assembly 

Sun  sensor 

Star  sensor 

Solar  array 

Radial  thruster 





Orbit  insertion  motor 


Aft  omni 

transmitter.  The  dish  antenna  transmitted 
both  X-  and  S-band  signals.  Mission  scientists 
could  direct  this  antenna  to  point  15°  from 
Earth-line  as  the  Orbiter  passed  behind  Venus. 
As  the  radio  waves  passed  through  Venus' 
atmosphere,  they  were  refracted,  or  bent, 
toward  Earth.  Without  repositioning  the 

antenna  again,  the  radio  signal  would  have 
been  refracted  away  from  Earth.  Repointing 
the  antenna  allowed  the  radio  beam  to  dip  deeply 
into  Venus'  atmosphere  and  still  reach  Earth 
despite  refraction  by  the  Venusian  atmosphere. 
Radio  occultation  data  were  thus  obtained  at 
atmosphere  levels  closer  to  the  planet's  surface. 


Reg  power, 


figure  3-?  3.  Tri/s 
diagram  of  the 
system  for  the 
common  Bus  on 
the  Orbiter  and 
Multiprobe  shows 
how  the  antennas 
could  connect  to 
the  redundant 
receivers  and  the 
power  amplifiers. 


The  X-band  signal  could  not  be  modulated,  so 
it  was  used  solely  to  study  atmospheric  effects 
on  radio  signals  at  two  different  frequencies. 
These  studies  provided  many  details  about  the 
planet's  atmosphere. 

Communications  from  Earth 
Regardless  of  its  orientation,  the  Orbiter  could 
receive  commands  from  Earth.  It  was  able  to 
do  this  through  two  redundant  S-band  tran- 
sponders connected  to  its  omnidirectional 
antennas.  Each  transponder  received  the  radio 


signal  from  Earth.  It  then  tuned  the  transmitter 
so  that  the  outgoing  radio  signals'  frequency 
from  the  spacecraft  had  a  constant  ratio  to  the 
incoming  signals'  frequency.  This  created  a 
coherent  mode  of  transponder  operation.  The 
coherence,  in  turn,  allowed  the  system  to 
measure  precisely  the  Doppler  shift  in  the 
radio  frequency  arising  from  the  spacecraft's 
motion  relative  to  Earth.  This  measurement 
was  possible  both  on  the  outgoing  and 
incoming  radio  signals.  Thus,  the  spacecraft's 
velocity  could  be  measured  to  3  m/hr. 

The  receiver  portion  of  each  transponder 
responded  only  to  certain  frequencies.  If  they 
did  not  receive  a  command  from  Earth  within 
36  hours,  the  receivers  automatically  reversed. 
Thus,  if  one  receiver  failed,  the  other  auto- 
matically took  over  within  36  hours. 

The  uplink  (Earth-to-spacecraft)  command 
capability  was  maintained  by  modulating  the 
S-band  carrier  of  approximately  2.115  GHz. 
The  down-link  telemetry  modulated  an  S-band 
carrier  of  approximately  2.295  GHz. 


The  Orbiter's  power  subsystem  provided  a 
semiregulated,  28  V  direct  current  to  all  its 
electrical  loads,  including  its  science  instru- 
ments. The  primary  source  of  power  was  the 
solar  array,  which  had  7.4  m2  (80  ft2)  of  solar 
cells.  Each  cell  was  2  cm2  (0.79  in.2).  At  Earth's 
orbit,  the  solar  array  provided  226  W,  and  at 
Venus  it  provided  312  W.  When  the  array's 
output  was  insufficient,  two  nickel-cadmium 
batteries  began  operating  automatically.  This 
occurred  when  the  bus  voltage  dropped  below 
27.8  V.  (Passing  through  Venus'  shadow  or  an 
inadequate  angle  of  sunlight  on  the  solar  array 
could  cause  these  voltage  drops.)  Each  battery 
was  rated  at  7.5  A  hr,  and  small  solar  arrays 
recharged  the  units.  Seven  shunt  limiters 

dissipated  excess  solar  power.  This  precaution 
kept  the  bus  voltage  at  30  V  or  below. 

A  power  interface  unit  switched  power  to  the 
Orbiter's  propulsion  unit  heaters  and  other 
heaters  as  they  needed  it.  This  interface  unit 
contained  protective  fuses.  Power  was  distrib- 
uted through  the  spacecraft  on  four  separate 
power  buses.  If  more  current  started  to  flow 
than  was  safe,  the  system  removed  loads  to 
prevent  a  catastrophic  failure.  First,  it  discon- 
nected the  scientific  instruments.  Then  it 
disconnected  the  switched  loads,  such  as 
control  and  data-handling  units.  The  transmit- 
ter was  the  final  unit  it  took  off  line.  The 
system  left  in  a  continuous  power-on  mode 
only  those  loads  that  were  absolutely  essential 
for  the  spacecraft.  These  loads  included  the 
command  units,  heaters,  receivers,  and  power 
conditioning  units. 

Multiprobe  Spacecraft 

The  Multiprobe  (Figure  3-14)  consisted  of  a 
basic  Bus  similar  to  the  Orbiter's,  a  Large 
Probe,  and  three  identical  Small  Probes.  It  did 
not  carry  a  despun,  high-gain  antenna.  The 
weight  of  the  Multiprobe  was  875  kg  (1930  Ib), 
including  32  kg  (70  Ib)  of  hydrazine.  The 
Multiprobe  used  this  propellant  to  correct  its 
trajectory  and  orient  its  spin  axis.  The  total 
weight  of  the  four  probes  it  carried  was 
585  kg  (1289  Ib).  The  Bus  itself  weighed 
290  kg  (639  Ib). 

The  Multiprobe's  basic  Bus  design  was  similar 
to  the  Orbiter's  design.  It  also  used  a  number 
of  common  subsystem  designs.  Mechanically, 
the  Bus  consisted  of  five  subassemblies:  (1)  a 
support  structure  for  the  Large  Probe,  (2)  a 
support  structure  for  the  Small  Probes,  (3)  an 
equipment  shelf,  (4)  a  solar  array  around  the 
periphery  of  the  cylindrical  basic  Bus,  and 
(5)  a  central  thrust  tube.  The  spacecraft 





Small  Probe 
interface  — 

Large  Probe 



Probe  1  - 



Fwd  omni 




Small  Probe 
release  clamp 




—   Fwd  thermal  barrier 
Equipment  shelf 

Radial  thruster 

Radial  thruster 
cutout  (4) 

Aft  omni 




tank  (2) 


Medium  gain 
horn  antenna 

Shelf  support 
strut  (12) 

L  Aft  axial  thruster 

Note:  Star  sensor,  thrusters,  aft  omni  antenna,  propellent 
tanks,  and  horn  antenna  rotated  for  clarity 

Figure  3-1 4.  The  Multiprobe 
spacecraft.  (Top)  General 
view  showing  major  parts. 
(Bottom)  Detailed  cross  section 
and  side  view  identifying 
major  components. 


Figure  3-75.  During  the  flight 
to  Venus,  the  Multiprobe  Bus 
carried  the  four  probes  in 
this  configuration. 

diameter  was  2.5  m  (8.3  ft).  From  the  bottom 
of  the  Bus  to  the  top  of  the  Large  Probe 
mounted  on  it,  the  Multiprobe  measured  2.9  m 
(9.5  ft). 

During  their  flight  to  Venus,  the  four  probes 
were  carried  on  a  large  inverted  cone  structure 
and  three  equally  spaced  circular  clamps 
surrounded  the  cone  (Figure  3-15).  Bolts  held 


these  attachment  structures  to  the  control 
thrust  tube.  This  thrust  tube  formed  the  struc- 
tural link  to  the  launch  vehicle.  The  Large 
Probe  was  centered  on  the  spin  axis.  A 
pyrotechnic-spring  separation  system 
launched  the  probe  from  the  Bus  toward  Venus. 
The  ring  support  clamps  that  attached  the 
Small  Probes  were  hinged.  To  launch  the 
Small  Probes,  the  Multiprobe  first  spun  up  to 
45  rpm.  Then  explosive  nuts  fired  to  open  the 
clamps  on  their  hinges.  This  sequence  allowed 
the  probes  to  spin  off  the  Bus  tangentially. 

The  forward  omnidirectional  antenna  of  the 
Multiprobe  extended  above  the  top  of  the  Bus 
cylinder.  An  aft  omni  antenna  extended  below 
it.  Both  these  antennas  had  hemispherical 
radiation  patterns.  A  medium-gain  horn 
antenna  was  on  the  instrument  shelf  and 
radiated  aft  of  the  spacecraft.  Ground  control 
used  it  during  critical  maneuvers  when  the  aft 
of  the  spacecraft  pointed  toward  Earth  as  the 
probes  separated  from  the  Bus. 

The  instrument-equipment  compartment,  as 
in  the  Orbiter,  carried  the  scientific  experi- 
ments and  electronics  for  the  spacecraft 
subsystems.  The  solar  array  provided  electrical 
power  from  solar  radiation.  It  contained  the 
batteries  and  a  power  distribution  system,  Sun 
and  star  sensors,  propellant  storage  tanks,  and 
thrusters  for  maneuvering  and  stabilization. 
The  Bus  also  carried  radio  transmitters  and 
receivers,  data  processors,  and  a  command  and 
data  handling  system. 

The  thermal  design  was  essentially  the  same  as 
the  Orbiter's.  In  addition,  the  Bus  required 
protective  surfaces  near  the  Small  Probes. 
These  surfaces  kept  the  probes  at  the  required 
temperature  during  the  cruise.  They  also 
protected  the  Bus  itself  from  heating  after  the 
probes  had  separated  from  it. 

Except  for  not  having  to  position  a  high-gain 
antenna,  orientation  controls  for  the 
Multiprobe  were  the  same  as  the  Orbiter's.  The 
propulsion  system  also  was  identical  to  the 
Orbiter's  with  one  exception:  the  Multiprobe 
only  had  one  aft  axial  thruster.  The  spacecraft 
did  not,  of  course,  carry  a  retrorocket. 

Data-Handling  System 
The  Multiprobe's  data-handling  system  was 
virtually  identical  to  Orbiter's.  The  only 
difference  was  its  lack  of  data  memory. 

Data  formats  were  organized  for  the 
Multiprobe's  special  mission  requirements. 
Before  the  probes  separated  from  the  Bus,  the 
Multiprobe  handled  data  for  the  Bus  and  all 
probes.  After  separation,  the  probes  used  their 
own  data  systems. 

The  Multiprobe's  data  system  accepted  engi- 
neering and  selected  information  that  mission 
operations  required  and  information  from  the 
four  probes.  It  also  accepted  data  from  the 
Multiprobe  Bus  itself  and  from  the  experi- 
ments on  the  Bus.  It  converted  analog  data  to 
digital  form  and  prepared  all  information  for 
transmission  to  Earth.  Each  telemetry  major 
frame  contained  64  minor  frames  composed  of 
64  eight-bit  words.  The  system  arranged  these 
words  in  several  formats.  Each  minor  frame 
contained  high-rate  science  or  engineering 
data,  plus  subcommutated  data,  spacecraft 
data,  and  frame  synchronization  data.  One 
subcommutated  format  carried  low  bit-rate 
science  and  science  housekeeping  information; 
two  were  for  low  bit-rate  information  from  the 
spacecraft  subsystems.  The  system  used 
12  real-time  data  transmission  rates  between 
8  and  2048  bits/sec.  Like  the  Orbiter,  the 
Multiprobe  also  had  high  bit-rate  formats  for 
attitude  control  during  maneuvers,  for  engi- 
neering data,  and  for  reading  out  the  contents 
of  the  command  memory.  A  single  format  for 
use  during  entry  into  the  Venus  atmosphere 
transmitted  science  data  at  1024  bits/sec. 

Command,  Communications, 
and  Power 

The  Multiprobe's  command  and  communica- 
tions subsystems  were  similar  to  the  Orbiter's. 
The  command  subsystem  decoded  all  com- 
mands received  via  the  Multiprobe's  communi- 
cations subsystem  at  a  fixed  rate  of  4  bits/sec. 
The  subsystem  could  either  store  these  com- 
mands for  later  execution  or  route  the  com- 
mands as  they  reached  their  destination  within 

the  spacecraft  and  the  probes  where  they  were 
implemented.  The  communications  subsystem 
provided  reception  and  transmission  for  radio 
communications  from  and  to  Earth 
(Figure  3-16). 

Also,  the  Multiprobe's  power  system  was  essen- 
tially the  same  as  the  Orbiter's.  One  difference, 
however,  was  a  power  interface  unit  that  could 
send  power  to  the  probe  heaters  and  the  probe 
checkout  buses,  and  to  relay  drivers  for  each  of 
the  probes.  This  system  allowed  the  probes  to 
receive  power  from  the  Bus  without  depleting 
their  own  batteries  during  the  interplanetary 
cruise  to  Venus.  The  Multiprobe's  solar  array, 
consisting  of  6.9  m2  (74  ft2)  of  2  cm  x  2  cm 
cells,  provided  214  W  near  Earth  and  241  W 
at  Venus. 

The  Probes 

The  probes'  designers  faced  a  number  of 
tremendous  challenges.  Among  them — the 
high  pressure  in  the  lower  regions  of  Venus' 
atmosphere,  which  was  about  100  times 
greater  than  Earth's  atmospheric  pressure  at 
sea  level;  the  high  temperature  of  about  480°C 
(900°F)  at  the  surface;  and,  the  corrosive 
constituents  of  the  clouds,  such  as  sulfuric 
acid.  Moreover,  these  probes  had  to  enter  the 
atmosphere  at  a  speed  of  about  41,600  km/hr 
(25,850  mph),  or  43  times  the  speed  of  a 
typical  commercial  jet. 

The  Large  and  Small  Probes  were  similar  in 
shape.  The  main  component  of  each  probe 
was  a  spherical  pressure  vessel.  Machined  from 
titanium,  the  vessels  were  sealed  against  the 
vacuum  of  space  and  the  high  pressure  of 
Venus'  atmosphere.  Within  this  pressure  vessel 
were  scientific  instruments  and  various  sub- 
systems for  the  probe's  operation. 

An  outer  structure  surrounded  each  spherical 
pressure  vessel.  This  structure  consisted  of  a 


From  bus 
via  IFD 

COM  power 
and  control 

COM  power  off 
discrete  command 

Coast  timer  telemetry    ^ 
to  bus  via  IFD 

Telemetry  control 

(read  clock  and  envelope) 

Coast  timer  set 
quantitative  command 

Coast  timer  start     

discrete  command 

Command  output 
inhibit  signal 

switch  (TSW)* 

switch  (ASW) 

Telemetry  data 

from  scientific 

instruments  and 

probe  subsystems 

34  analog 

24  bilevel 

12  serial  digital** 

2  low  level  analog 

*Large  Probe  only 
"Includes  2  internal  channels 





data  unit 

—  v 

To  scientific 
•  instruments  and 
probe  subsystems 

_  Telemetry  subcarrier  to  bus 
and  probe  transponders 

Coast  timer  pretimeout 
and  timeout  commands 

_^_  Timer  signals  to 
science  instruments 

_K  Discrete 

+28      +15 

36  squib 



Figure  3-16.  The  main 
components  of  the  command 
data  subsystem  for  the  Multi- 
probe  spacecraft  are  related  in 
this  block  diagram. 


conical  aeroshell  and  an  aft  shield.  The 
aeroshell,  shaped  as  a  45°  cone  with  a 
hemispherical  blunt  top,  was  a  one-piece 
aluminum  structure  with  integrally  machined 
stiffening  rings.  The  aeroshell's  heat  shield 
protected  the  probe  from  the  heat  of  a  high- 
speed atmospheric  entry.  The  aeroshell  also 
acted  aerodynamically  to  keep  the  probe  stable 
on  its  flight  into  the  atmosphere.  The  aft  cover 
of  fiberglass  honeycomb  had  a  Teflon  flat 
section  transparent  to  radio  waves.  It  protected 
the  aft  hemisphere  of  the  pressure  vessel 
during  entry  into  the  Venusian  atmosphere. 
Spin  vanes  kept  the  probes  spinning  during 
descent  to  maintain  stability. 

All  instruments  within  the  probes'  pressure 
vessels  required  either  observations  or  direct 

sampling  of  Venus'  hostile  atmosphere. 
Providing  such  access  was  a  major  design 
problem.  The  Large  Probe  had  to  have 
14  sealed  penetrations  through  the  walls  of  its 
pressure  vessel:  one  for  the  antenna,  four  for 
electrical  cables,  two  for  access  hatches,  and 
seven  for  scientific  instruments.  Each  Small 
Probe  required  seven  such  penetrations:  one 
for  the  antenna,  three  for  electrical  cables, 
one  for  an  access  hatch,  and  two  for  scientific 
instruments.  Special  diamond  and  sapphire 
windows  admitted  light  or  heat  at  wave- 
lengths required  for  several  of  the  science 

The  Large  Probe 

The  Large  Probe  (Figure  3-17)  weighed  about 
315  kg  (695  Ib)  and  was  about  1.5  m  (5  ft)  in 




mod  umbilical 
cable  cutter 

Aft  cover 

Parachute  tower 

Solar  flux 



Neutral  mass 
spectrometer  inlet 


Cut  out  for 
temperature  sensor 
atmosphere  structure 

Cloud  particle 

Descent  module 

Spin  vanes 

Pressure  vessel 
separation  assembly 


Pilot  chute 
and  mortar 

Probe/bus  in-flight 

Deceleration  module 

Figure  3- 1 7.  Detail  of  the  Large 
Probe,  including  pressure  vessel, 
protective  nose  cone,  aft  shield, 
and  major  components. 

diameter.  It  consisted  of  a  forward  aeroshell 
heat  shield,  a  pressure  vessel,  and  an  aft  cover. 
Precisely  machined  from  titanium  to  achieve 
high  strength  at  high  temperatures  and  still  be 
lightweight,  the  pressure  vessel  (Figure  3-18) 
was  73.2  cm  (28.8  in.)  in  diameter.  It  was  made 
in  three  flanged  pieces:  an  aft  hemisphere,  a 
flat  ring  section,  and  a  forward  cap.  These  were 
bolted  together  with  seals  between  the  flanges. 
The  seals  were  a  combination  of  two  elements. 
The  first  were  O-rings  to  prevent  leakage  of  the 
probe's  102  kPa  (15  psia)  nitrogen  atmosphere 
during  transit  to  Venus.  The  second  were 
Graphoil  flat  gaskets  to  prevent  inward  leakage 
of  Venus'  hot  atmosphere  during  descent  to 

the  surface.  A  pressure  bottle  was  on  the 
forward  shelf  of  the  Large  Probe.  A  stored 
command  fired  the  bottle,  which  increased  the 
probe's  internal  pressure  by  41  kPa  (6  psi). 
Inside  the  pressure  vessel,  two  parallel  beryl- 
lium shelves  served  as  supports  and  heat 
absorbers  for  the  instruments  and  spacecraft 
systems  mounted  on  them.  A  2.5-cm  (1-in.) 
thick  blanket  of  multilayered  Kapton,  which 
completely  lined  the  interior,  further  protected 
equipment  inside  the  pressure  shell  from  heat 
encountered  at  Venus. 

Four  scientific  instruments  used  nine  observa- 
tion windows  through  four  of  the  pressure 


Figure  3-  7  8.  Arrangement  of 
scientific  instruments  and  other 
spacecraft  components  in  the 
Large  Probe's  pressure  vessel. 



Pressure  vessel 
aft  section 

Coolant  port 

Access  hatch 

Insulation  blanket 

Driver  amplifier 


Pressure  vessel 

control  units 


vessel  penetrations.  Eight  windows  were 
sapphire  and  one  was  diamond.  Three  vessel 
penetrations  were  inlets  for  direct  atmospheric 
sampling  by  a  mass  spectrometer,  a  gas 
chromatograph,  and  an  atmospheric  structure 
experiment.  At  the  aft  pole  of  the  pressure 
vessel,  an  antenna  had  a  hemispherical  radia- 
tion pattern.  This  provided  communications 
with  Earth  when  the  probe  had  separated  from 
its  Bus.  Extending  10  cm  (4  in.)  on  one  side  of 
the  pressure  vessel,  two  arms  held  a  reflecting 
prism  for  cloud  particle  observations.  On  the 
opposite  side  of  the  pressure  vessel,  a  single 
arm  carried  a  temperature  sensor  on  its  tip. 

Three  parachute-shroud  towers  were  mounted 
above  aerodynamic  drag  plates.  These  plates 
were  at  equal  distances  around  the  spherical 
vessel's  equator.  Of  two  access  ports,  one  was 
for  electronic  checkout  of  the  system  before 
launch.  The  other  provided  a  cooling  port  that 
scientists  also  used  during  ground  tests. 

During  high-speed  entry  into  Venus'  atmo- 
sphere, a  carbon  phenolic  ablative  heat  shield 
protected  the  Large  Probe  from  overheating. 
This  shield  was  bonded  to  and  covered  the 
outer  surface  of  the  forward-facing  aeroshell. 
All  other  surfaces  of  the  aeroshell  and  the  aft 
cover  were  coated  with  a  heat-resisting, 
low-density,  elastomeric  material. 

The  Large  Probe  performed  a  fixed  sequence  of 
operations  when  it  reached  Venus.  Communi- 
cations with  Earth  started  22  minutes  before 
entry  into  Venus'  atmosphere.  A  peak  decel- 
eration of  280  g  occurred  soon  after  entry, 
and  the  spacecraft  jettisoned  the  aft  cover  to 
deploy  a  parachute.  A  mortar  fired  a  pilot 
chute  from  a  small  compartment  in  the  side  of 
the  aeroshell.  Lines  attached  this  parachute  to 
the  aft  cover,  which  an  explosive  bolt  sepa- 
rated so  that  it  could  then  pull  free.  The  cover, 
in  turn,  was  attached  to  the  main  parachute. 
The  pilot  chute  then  pulled  the  main  chute 
from  its  compartment  within  the  conical 


chute  bag 

Deploy  main 



Parachute  jettison 


10  15  20  25  30  35  40  45  50  55 
Time  (min) 

Figure  3- 1 9.  (Top)  The  release 
sequence  for  the  Large  Probe's 
parachute  appears  in  these 
drawings.  (Bottom)  This  graph 
displays  altitude  plotted  against 
time  for  the  Large  and  Small 
Probes.  Both  Large  and  Small 
Probes  took  about  the  same 
time  to  reach  Venus'  surface. 

aeroshell.  As  soon  as  the  spacecraft  was 
stable,  explosive  nuts  or  cable  cutters  severed 
mechanical  and  electrical  ties  to  the  aeroshell. 
The  main  chute  then  pulled  the  pressure  vessel 
free  from  the  aeroshell  (Figure  3-19). 

ing  the  atmosphere.  Spin  vanes  around  the 
pressure  vessel  spun  it  at  less  than  1  rpm  dur- 
ing its  descent.  A  forward-facing  aerofairing,  a 
conical  skirt,  and  sectional  drag  plates  kept  the 
spacecraft  stable. 



The  heat  shield  jettisoned  about  67  km 
(42  miles)  above  the  surface.  About  47  km 
(29  miles)  above  the  surface,  the  parachute 
released.  The  probe  fell  freely  so  that  it  reached 
the  surface  about  55  minutes  after  first  enter- 

Communications  Subsystem 
The  Large  Probe's  communications  subsystem 
(Figure  3-20)  had  a  solid  state  transmitter  to 
return  a  data  stream  directly  to  Earth  at 
256  bits/sec.  Four  10-W  amplifiers  provided 

a  transmitter  power  of  40  W.  A  transponder 
received  an  S-band  carrier  from  Earth  at 
2.1  GHz.  It  set  the  probe's  transmitter  to  send 
at  2.3  GHz  from  the  crossed  dipole  antenna 
on  the  aft  hemisphere.  The  mission  used  the 
transponder  receiver  for  two-way  Doppler 
tracking  only.  The  incoming  signal  carried 
no  information,  and  the  Large  Probe  did  not 
receive  commands  from  Earth. 

Power  Subsystem 

A  40  A  hr,  silver-zinc  battery  powered  the 

probe.  During  descent,  the  spacecraft  main- 

tained output  at  28  V  direct  current.  The 
power  system  consisted  of  the  battery,  a  power 
interface  unit,  and  a  current  sensor.  Before  the 
probe  separated  from  the  Bus,  it  received 
power  from  the  Bus  for  checking  and  heating 
the  probe.  During  this  time,  the  internal 
battery  was  open-circuited  by  switches  in  the 
probe's  power  interface  unit. 

Command  Subsystem 

Once  the  Large  Probe  had  separated  from  the 
Bus,  its  internal  electronics  provided  all  com- 
mands for  its  operation.  The  command 

Figure  3-20.  This  block  diagram 
shows  interconnections  among 
communication  subsystem 
components  for  the  Large  Probe. 



TLM  SC  Rtn 
Exciter  on 




1 . -in  denotes  (n)  inter- 
connecting lines  shown 
as  single  line  group 

28  V  (rcvr)  is  separately 
switched  on  by  power 
interface  unit  for  receiver  on 


3.  All  other  28  V  lines  are  switched 
on  together  by  power  interface 
unit  for  RF  on 

subsystem  consisted  of  a  command  unit,  a 
pyrotechnic  control  unit,  and  sensors  to 
service  the  command  unit,  including  sensors 
to  measure  the  probe's  deceleration.  The 
internal  command  subsystem  had  64  separate 
commands  for  the  spacecraft  itself  and  for  its 
payload  of  scientific  instruments.  It  contained 
a  coast  timer  that  was  the  spacecraft's  only 
instrument  working  during  separation  from 
the  Bus  to  entry  into  Venus'  atmosphere. 
During  this  period,  all  other  probe  subsystems 
were  off.  There  also  was  an  entry  sequence 
programmer  and  a  command  decoder.  The 
entry  sequence  programmer  transmitted 
53  discrete  commands  in  a  fixed  sequence.  Two 
instruments  could  start  these  commands:  the 
coast  timer  or  an  accelerometer  switch  that 
sensed  the  deceleration  of  entry.  A  temperature 
switch  acted  as  back-up  for  the  timer  when  the 
parachute  jettisoned. 

Pyrotechnic  Control  Unit 
The  pyrotechnic  control  unit  consisted  of 
12  squib  drivers.  These  drivers  provided  cur- 
rent to  fire  explosive  nuts  to  release  the  aero- 
shell,  the  aft  cover,  and  the  parachute.  There 
also  were  actuators  for  the  cable  cutter,  the 
pilot  chute  mortar,  and  the  instrument  that 
released  the  protective  cover  of  the  mass 
spectrometer  inlet  port. 

Data-Handling  Subsystem 
The  Large  Probe's  data  subsystem  handled 
36  analog,  12  serial  digital,  and  24  bilevel 
status  channels  from  scientific  instruments  and 
subsystems  within  the  probe.  The  unit  con- 
verted all  data  into  major  telemetry  frames. 
These  frames  consisted  of  16  minor  frames  for 
time-multiplexed  transmission  to  Earth.  Each 
minor  frame,  in  turn,  consisted  of  a  series  of 
64  8-bit  words  for  a  total  of  512  data  bits  per 
minor  frame. 

The  data  handling  subsystem  provided  two 
data  formats:  one  for  use  during  radio  blackout 
caused  by  the  plasma  sheath  during  entry,  the 
other  for  use  during  normal  descent  after  the 
probe  slowed  down.  There  was  a  solid-state 
memory  with  a  storage  capacity  of  3072  bits. 
This  storage  allowed  the  probe  to  store  data 
during  communications  blackout  and  transmit 
it  afterwards.  The  system  stored  data  in  memory 
at  128  bits/sec  but  read  it  out  at  256  bits/sec. 
This  was  the  normal  bit-rate  for  data  transmis- 
sion to  Earth  during  the  descent.  For  5  minutes 
before  entry  to  30  seconds  after  entry,  the 
transmission  bit-rate  was  only  128  bits/sec. 
The  full  bit-rate  was  allocated  among  the  seven 
experiments  at  16  to  44  bits/sec  for  each  experi- 
ment. The  nephelometer  and  atmospheric 
structure  experiments,  however,  were  able  to  use 
the  blackout  storage  format  of  4  and  72  bits/sec, 
respectively.  Two  subcommutated  formats  for 
low  bit-rate  phenomena  also  provided  data  for 
housekeeping  and  for  the  atmospheric  struc- 
ture, nephelometer,  cloud  particle  spectrom- 
eter, and  solar  flux  radiometer  experiments. 

The  Small  Probes 

The  three  Small  Probes  (Figure  3-21)  were 
identical.  In  contrast  to  the  Large  Probe,  they 
did  not  carry  parachutes.  Aerodynamic  brak- 
ing slowed  them  down.  Like  the  Large  Probe, 
each  Small  Probe  consisted  of  a  forward  heat 
shield,  a  pressure  vessel,  and  an  afterbody. 
The  heat  shield  and  the  afterbody  remained 
attached  to  the  pressure  vessel  all  the  way  to 
the  surface.  Each  probe  was  0.8  m  (30  in.)  in 
diameter  and  weighed  90  kg  (200  Ib). 

Engineers  precisely  machined  the  pressure 
vessel  (Figure  3-22)  from  titanium  in  two 
flanged  hemispheres.  Bolts  joined  the  hemi- 
spheres with  seals  between  the  flanges.  The 
vessel  nested  within  the  aeroshell  and  was 
permanently  attached  to  it.  The  Small  Probes 
used  two  types  of  seals  similar  to  the  Large 


Figure  3-21.  The  three  Small 
Probes  were  identical.  Major 
components  are  identified. 

Antenna  housing 

Net  flux 

Probe/bus  interface 

ring  for 

separation  clamp 

Nephelometer  door 
(shown  closed) 

Deceleration  Module 

Ground  coolant 
access  cover 

structure  door 


structure  pressure 
inlet  and  spin 
control  vane 

Yo-yo  cable  cutter 
Yo-yo  despin  weight 

Yo-yo  despin  cable 

Carbon  phenolic 
heat  shield 


Probe's:  (1)  O-rings  to  maintain  internal  pres- 
sure and  (2)  Graphoil  flat  gaskets  to  prevent 
Venus'  hot  atmosphere  from  leaking  in.  The 
afterbody  also  was  a  permanent  part  of  the  pres- 
sure vessel,  its  shape  closely  matching  the 
pressure  vessel's.  Xenon  filled  each  Small 
Probe's  interior  at  a  pressure  of  approximately 
102  kPa  (15  psia).  Engineers  used  xenon  instead 
of  nitrogen — they  used  nitrogen  in  the  Large 
Probe — to  reduce  heat  flow  from  the  pressure 
vessel  walls  to  instruments  and  probe  space- 
craft systems.  As  in  the  Large  Probe,  a  protective 
blanket  lining  of  Kapton  further  slowed  this  flow. 
Instruments  and  spacecraft  subsystems  were  on 
two  beryllium  shelves  that  absorbed  heat. 

The  aeroshell  had  the  same  basic  45°,  blunt 
cone  design  as  the  Large  Probe.  It,  too,  used  a 
bonded  carbon  phenolic  ablative  coating  as  a 
heat  shield.  Because  the  shield  had  to  protect 
the  pressure  shell  down  to  the  surface,  engi- 
neers made  the  aeroshell  from  titanium  (the 
Large  Probe  had  an  aluminum  aeroshell).  The 

shell  had  a  stressed  skin,  or  monocoque  (one 
piece),  construction. 

The  Small  Probes'  entry  sequence  started  with 
communications  22  minutes  before  entry. 
About  5  minutes  before  entry,  a  pyrotechnic 
cable  cutter  cut  two  weights  loose.  The 
weights  were  now  free  to  swing  out  like  yo-yos 
on  2.4-m  (8-ft)  cables.  As  a  result,  each 
probe's  spin  rate  slowed  from  about  48  rpm 
to  17  rpm.  The  probes  then  jettisoned  the 
weights  and  cables.  This  reduction  in  spin  rate 
allowed  aerodynamic  forces  to  line  up  the 
probes.  Now  their  heat  shields  could  protect 
them  from  the  heating  of  entry.  All  probes 
entered  the  atmosphere  at  a  speed  of  about 
42,000  km/hr  (26,099  mph).  The  probe 
making  the  steepest  entry  underwent  a  peak 
deceleration  of  458  g,  the  others  somewhat 
less.  The  probe  making  the  shallowest  entry 
decelerated  the  least  at  about  223  g.  Three 
doors  on  the  afterbody  then  opened  at  an 
altitude  of  about  70  km  (43  miles)  to  give 


Aft  insulation 

Pressure  vessel 
aft  section 


data  unit 






Forward  shelf 



Aft  shelf 




Pressure  vessel 
forward  section 

Figure  3-22.  A  titanium  pressure 
vessel  carried  scientific  instru- 
ments and  spacecraft  systems 
for  each  Small  Probe.  The  Large 
Probe  used  the  same  type  of 
pressure  vessel. 

three  instruments  access  to  the  atmosphere. 
Two  of  the  doors  opened  from  each  of  two 
protective  housings.  One  was  for  the  atmo- 
spheric structure  experiment  and  the  other 
for  the  net  flux  radiometer  experiment.  The 
housings  projected  like  ears  from  each  side  of 
the  pressure  vessel's  sphere.  The  temperature 
sensor  and  atmospheric  pressure  inlet  for  the 
atmospheric  structure  instrument  extended 
10  cm  (4  in.)  from  the  door  of  one  housing. 
The  net  flux  radiometer  sensor  extended 
similarly  on  the  opposite  side. 

When  the  housing  doors  opened  after  atmo- 
spheric entry,  they  slowed  the  spacecrafts'  spin 
rate  because  they  did  not  jettison.  However,  a 
small  vane  on  the  pressure  sensor  inlet  kept 
the  spacecraft  spinning  throughout  its  descent. 
This  spin  allowed  the  instruments  to  scan 
around  the  probe.  A  cover  over  the  nephelom- 

eter  folded  down  after  it  opened.  Each  Small 
Probe  fell  freely  for  about  53  to  55  minutes 
until  it  reached  Venus'  surface. 

Communications  equipment  for  each  Small 
Probe  (Figure  3-23)  consisted  of  a  solid  state 
transmitter  and  a  hemispherical-coverage 
antenna,  similar  to  the  Large  Probe.  The 
antenna  was  on  the  aft  pole  of  the  pressure 
vessel  sphere  and  radiated  through  a  Teflon 
window.  Each  transmitter  had  one  10-W 
amplifier,  which  was  one-quarter  the  power  of 
the  Large  Probe's  transmitter.  Until  the  probes 
penetrated  to  roughly  30  km  (19  miles)  above 
Venus'  surface,  the  large  64-m  (210-ft)  anten- 
nas of  the  Deep  Space  Network  could  receive 
data  at  64  bits/sec.  From  there  on,  they  received 
data  at  16  bits/sec  only.  The  Small  Probes  did 
not  carry  a  receiver  for  two-way  Doppler 
tracking.  Instead,  each  probe  carried  a  stable 


figure  3-23.  Components  of  each 
Small  Probe's  communication 
subsystem  appear  in  this  block 
diagram.  The  subsystem  had 
only  one  power  amplifier  (the 
Large  Probe  had  four).  It  did 
not  have  a  receiver. 



3(2)         D(2)             Antenna 

i  i  v_y 

/         S 

/  2  /  2 

1  '      1  '      1  ' 

TLM  SC  Rtn 
'  '                                               Isolator 


<                           //^v\ 

0"             Off 


.?„                 v<y 

Power  amp 

A          t  i 
v          / 

/[  2         /  2 

L             t 

l                            i  ,                  A                    ' 
^                       x  2              /^ 

Reg  dwr 

/2  /|2 


Note:    /[2  denotes  (2)  interconnecting  lines  shown  as  single  line  group. 


oscillator.  This  provided  the  reference  fre- 
quency for  the  Doppler  measurements  that 
ground  control  used  in  its  computations. 

Each  probe  carried  an  1 1  A  hr  silver-zinc 
battery.  This  provided  28  V  direct  current 
during  the  descent.  As  with  the  Large  Probe, 
the  power  system  had  a  power  interface  unit 
and  a  current  sensor. 

The  command  subsystem  was  identical  to  the 
Large  Probe's.  No  uplink  (Earth-to-probe)  com- 
mand capability  existed.  After  separation,  all 
probe  commands  originated  from  their  respec- 
tive coast  timers,  programmers,  and  accelera- 
tion switches.  The  coast  timer  maintained 
control.  It  started  the  entry  sequence 

programmer,  which  transmitted  all  com- 
mands in  a  fixed  sequence.  The  transmission 
lasted  until  each  probe  impacted  with 
Venus'  surface. 

The  Small  Probe's  data-handling  subsystem 
components  were  the  same  as  the  Large 
Probe's.  Each  probe  used  three  major  data 
formats:  upper  descent,  blackout,  and  lower 
descent.  These  formats  contained  16  minor 
frames  of  64  eight-bit  words.  As  on  the  Large 
Probe,  a  3072-bit  solid  state  memory  stored 
data  when  communications  with  Earth  were 
blacked  out  by  a  plasma  sheath  on  entry. 
These  data  were  transmitted  after  the  probes 
had  slowed  down  and  the  plasma  sheath 
had  dissipated. 





With  this  anagram  in  1610,  Galileo  reported 
his  first  scientific  observation  of  Venus.  His 
observation  broke  centuries  of  failure  to  see 
what,  in  retrospect,  is  obvious:  Earth  is  not  the 
center  of  the  Universe.  Galileo's  message, 
when  unscrambled  and  translated  into 
English,  said,  The  mother  of  the  loves  emulates 
the  phases  of  Cynthia.  That  is,  Venus  exhibits 
phases  like  the  Moon. 

In  the  centuries  that  followed  Galileo's 
observation,  scientists  made  many  more 
discoveries  about  the  cloud-shrouded  planet. 
Yet,  there  were  equally  as  many  speculations 
about  Venus'  true  nature.  These  speculations 
ranged  from  a  dust-ridden  world  to  a  world  of 
swamps  to  one  in  a  sea  of  hydrocarbons. 
Modern  Earth-based  observations  disproved 
many  of  the  earlier  theories.  Such  observations 
used  highly  sophisticated  new  instruments 
and  data  reduction  techniques.  New  data  also 
came  from  several  Venus  flybys  and  some 
Russian  probes  that  had  landed  on  the 
surface.  Despite  this  work,  many  un- 
knowns about  Earth's  sister  planet  sti 

To  resolve  these  unknowns,  scientists  designed 
the  scientific  payloads  of  the  Pioneer  Venus 
Orbiter  and  Multiprobe  spacecraft  to  obtain 
new  information  about  Venus.  Six  spacecraft 
carried  advanced  scientific  instruments  that 
revised  our  notions  about  Venus.  These 
spacecraft  altered  our  understanding  as 
drastically  as  Galileo's  observations  changed 
many  of  his  contemporaries'  beliefs.  With 
Pioneer  Venus,  scientists  were  the  first  to  look 
globally  through  the  thick  cloud  layers.  They 
sampled  the  constituents  of  Venus'  dense 
atmosphere.  Also  they  made  long-term 
observations  of  changes  within  that  atmo- 
sphere and  of  its  ultraviolet  cloud  markings. 

New  viewpoints  resulted  from  the  Pioneer 
mission  to  Venus.  These  viewpoints  influenced 
comparative  planetologists  and  other  scientists 
as  they  worked  to  refine  theories  to  explain 
the  evolution  of  the  Solar  System  and  its 

The  Pioneer  Venus  project 
changed  forever  the  way  we 
looked  at  Venus.  The 
project's  foundations  were 
carefully  designed  investiga- 
tions and  engineered  instru- 
ments carried  by  two  space- 
craft, an  Orbiter  and  a 
Multiprobe.  The  Orbiter  had 
four  scientific  objectives: 
study  Venus'  upper  atmo- 
sphere and  ionosphere, 
clouds,  surface,  and  gravita- 
tional field.  The  Multiprobe's 
experiments  focused  on  the 
atmosphere  to  investigate  the 
components,  composition, 
structure,  thermal  balance, 
circulation  around  Venus, 
and  interaction  with  solar 
wind.  In  this  chapter,  you 
learn  how  mission  scientists 
achieved  these  objectives 
with  specific  instruments 
and  investigations. 



Orbiter  Scientific  Objectives 

The  Orbiter  explored  Venus  in  four  important 
ways.  First,  it  investigated  the  clouds  globally. 
To  do  this,  it  used  advanced  technology  sen- 
sors aboard  the  spacecraft.  It  also  observed 
how  Venus'  atmosphere  affected  radio  signals 
from  the  spacecraft  to  Earth  as  the  spacecraft 
was  occulted  by  Venus.  Second,  it  measured 
the  upper  atmosphere  and  ionosphere's 
features  over  the  entire  planet  and  detected 
how  the  solar  wind  interacts  with  the  iono- 
sphere. Third,  it  used  a  radar  instrument  to 
penetrate  the  Venusian  cloud  layers  and 
obtain  information  about  the  planet's  surface. 
Finally,  it  determined  the  general  shape  of 
Venus'  gravitational  field  and  detected  local 
anomalies  in  it  by  measuring  how  the  field 
affected  the  spacecraft's  orbit. 

To  achieve  its  science  objectives,  the  spacecraft 
carried  a  complement  of  12  scientific  instru- 
ments. Three  instruments  provided  informa- 
tion to  answer  basic  questions  about  how 
Venus  interacts  with  the  solar  wind.  A  magne- 
tometer measured  magnetic  fields.  A  plasma 
analyzer  measured  solar  wind.  An  electric  field 
detector  measured  electric  fields. 

An  ultraviolet  spectrometer  measured  the 
intensity  of  ultraviolet  radiation  at  various 
wavelengths.  Its  aim  was  to  check  how  sun- 
light reflects  and  scatters  off  clouds  and  haze 
layers  in  Venus'  atmosphere.  This  instrument 
also  detected  day  and  night  glows  in  the  upper 
atmosphere.  They  are  caused  by  solar  radiation 
acting  on  the  gases  there  and  recombination 
of  molecules  when  solar  radiation  is  absent  at 
night.  The  instrument  also  investigated  a 
hydrogen  gas  corona  surrounding  the  planet. 

An  infrared  radiometer  measured  radiation  at 
selected  wavelengths  within  the  infrared  part 
of  the  electromagnetic  spectrum.  It  was 
sensitive  to  the  atmosphere's  emitting  tem- 

perature at  several  levels.  The  instrument  also 
detected  and  mapped  both  water  vapor 
distribution  in  the  atmosphere  and  reflected 
solar  radiation. 

A  radar  mapper  penetrated  the  cloud  layers  to 
determine  surface  topography  and  surface 
scattering  properties.  This  instrument  revealed 
surface  details  that  cloud  layers  obscured.  By 
using  side-looking  mapping,  it  also  provided 
information  on  the  radar  brightness  of 
the  surface. 

An  ultraviolet  spin-scan  imager  mapped  the 
Venusian  clouds.  To  build  a  picture,  this 
instrument  made  a  series  of  narrow  scans 
across  Venus.  The  process  is  similar  to  the  way 
a  television  creates  a  picture  by  scanning  a 
series  of  lines  across  the  tube  face.  The 
spacecraft's  rotation  swept  the  viewpoint  of 
the  instrument  across  the  planet.  While  this 
was  happening,  the  spacecraft's  motion  along 
its  orbit  placed  the  scan  paths  side  by  side  to 
build  images.  The  spin-scan  imager  also 
measured  intensity  and  polarization  of  light 
reflected  from  Venus'  clouds.  When  operating 
in  a  polarimetry  mode,  it  provided  informa- 
tion about  size,  shape,  and  types  of  particles 
making  up  clouds  and  haze  layers. 

When  the  Orbiter  was  closest  to  Venus,  at 
orbit  periapsis,  it  passed  briefly  through  the 
ionosphere  and  upper  atmosphere.  During 
those  periods,  it  used  several  instruments.  One 
identified  the  atmosphere's  neutral,  uncharged 
particles.  Another  measured  composition  and 
concentration  of  positively  charged  thermal 
ions.  A  retarding  potential  analyzer  and 
electron  temperature  probe  also  were  aboard 
the  Orbiter.  These  instruments  measured  the 
abundances  of  charged  particles  in  the  iono- 
sphere and  in  layers  between  the  ionosphere 
and  the  region  of  the  solar  wind.  It  determined 
ion  composition  and  electron  and  ion  energy. 

The  Orbiter  also  carried  an  instrument  that 
was  not  connected  with  Venus  exploration. 
The  instrument  measured  gamma  ray  bursts 
coming  from  space.  Before  a  special  research 
spacecraft  probed  space  in  the  1980s,  scientists 
could  not  determine  the  source  of  these  rays. 
The  observation  platform  that  Pioneer  Venus 
provided  complemented  experiments  that 
scientists  were  conducting  near  Earth.  While 
en  route  to  and  in  orbit  around  Venus,  it 
presented  an  opportunity  to  obtain  another  set 
of  data.  Researchers  used  these  data  to  triangu- 
late with  Earth-Orbiter  observations  and  find 
each  source's  direction. 

Multiprobe  Scientific  Objectives 

The  Multiprobe  spacecraft  consisted  of  a  Bus 
and  probes  to  investigate  Venus'  atmosphere 
in  four  major  ways.  First,  its  instruments 
sampled  gases  and  particles  within  the  clouds 
to  establish  their  nature  and  composition. 
Second,  its  science  experiments  determined 
composition,  structure,  and  thermal  balance 
of  the  planet's  atmosphere,  by  direct  sampling 
and  measurements  of  radiation  from  high 
altitudes  down  to  the  surface.  Third,  observa- 
tions of  the  atmospheric  probes'  paths  checked 
how  the  atmosphere  circulates  about  the 
planet.  Fourth,  the  spacecraft  gathered  data  to 
further  investigate  how  the  planet  interacts 
with  solar  wind. 

To  achieve  these  science  objectives,  the 
Multiprobe  spacecraft  carried  18  scientific 
experiments.  These  included  two  aboard  the 
Bus,  three  on  each  of  the  three  identical  Small 
Probes,  and  seven  on  the  Large  Probe. 

One  instrument  on  the  Bus  was  a  neutral  mass 
spectrometer.  This  sophisticated  instrument 
measured  density  and  analyzed  gas  composi- 
tion in  the  upper  atmosphere.  The  other 
instrument  was  an  ion  mass  spectrometer  that 

was  identical  to  the  Orbiter's.  It  determined 
the  composition  of  thermal  ions  in  the  upper 
atmosphere  and  measured  their  concentration 
and  temperature. 

Each  Small  Probe  carried  an  instrument  to 
detect  and  measure  optical  properties  of 
particles  at  various  levels  in  Venus'  atmosphere. 
It  also  carried  an  instrument  complex  to 
measure  atmospheric  temperature  and  pres- 
sure. These  sensors  had  two  main  functions. 
First,  they  defined  the  properties  of  the 
atmosphere  and  clouds  from  an  altitude  of 
about  65  km  (40  miles).  Second,  they  enabled 
investigators  to  establish  the  probe's  altitude 
during  each  measurement.  A  third  device 
monitored  the  amount  of  sunlight  penetrating 
to  different  atmospheric  levels.  The  instru- 
ment also  measured  the  amount  of  planetary 
infrared  radiation  emitted  back  to  space. 

The  Large  Probe  carried  the  first  two  experi- 
ments, which  were  described  above,  to 
determine  atmospheric  and  cloud  structure.  In 
addition,  it  carried  a  neutral  mass  spectrometer 
to  measure  the  composition  of  the  neutral 
atmospheric  components.  It  took  measurements 
from  an  altitude  of  about  65  km  (40  miles)  to 
the  surface.  This  instrument  identified  vapors 
that  condense  to  form  Venus'  clouds.  It  also 
measured  the  number  of  rare  gas  isotopes  in 
the  atmosphere.  This  isotopic  measurement 
was  important  in  tracing  the  planet's  history 
and  atmospheric  evolution.  Another  instru- 
ment, the  gas  chromatograph,  measured  the 
abundances  of  atmospheric  gases. 

To  find  out  which  solar  radiation  penetrates 
the  atmosphere  and  reaches  ground  level,  the 
Large  Probe  included  yet  another  instrument. 
Such  measurements  are  important  to  our  under- 
standing of  why  Venus  is  so  much  hotter  than 
Earth.  A  separate  instrument  measured  the 
infrared  part  of  the  solar  radiation  flux  at  all 


Figure  4-1 .  Orbiter  cloud  photo- 
polarimeter (OCPP).  (Top) 
Diagram  identifying  components 
of  instrument's  optical  system, 
including  its  telescope,  filter/ 
retarder  wheel,  and  photo- 
diodes.  (Bottom)  Photograph 
of  complete  instrument. 


UV  enhanced  silicon  photodiode  imaging  channel  - 
UV  enhanced  silicon  photodiode  limb  scan  channel  - 
Limb  scan  filter  and  diagonal  reflector 

Primary  mirror 

Entrance  window  and 
secondary  mirror 

Imaging  filters  and  UV  enhanced 

diagonal  reflectors  silicon  photodiodes 

(bonded  to  back  photopolarimetry 

side  of  wheel)  channels 

Half-wave  retarders  covering 
three  22-1/2°  positions  per  spectral 
band  (0°,  45°,  90°  optical  rotation)  - 

Filter/retarder  wheel 
(16  active  positions) 

polarized  beams 

Wollaston  prism 
Calibration  lamp 

Calibration  scattering 
surfaces  (back  side) 

-Photopolarimetry  spectral  filters 
back  side  of  wheel  (three  each  for 
four  spectral  bands) 

levels  in  the  atmosphere.  It  also  detected  the 
presence  of  clouds  and  water  vapor.  Another 
instrument  measured  particle  sizes  in  clouds 
and  in  the  lower  atmosphere,  and  determined 
particle  concentrations  at  various  levels. 

Earth  stations  received  radio  signals  from  all 
probes  and  the  Multiprobe  Bus.  Science  inves- 
tigators used  these  signals  to  make  extremely 
accurate  measurements  of  the  various  probes' 
velocities.  From  these  measurements,  research- 
ers calculated  wind  speeds  and  circulation 
patterns  in  Venus'  atmosphere. 

Orbiter  Instruments  and  Experiments 

Cloud  photopolarimeter 
The  photopolarimeter  measured  distribution 
of  cloud  and  haze  particles  and  detected 
ultraviolet  markings  and  cloud  circulations. 
The  ultraviolet  images  from  this  instrument 
provided  visual  references  for  data  from  other 
Orbiter  experiments  and  for  its  own  polariza- 
tion readings.  Principal  investigator  for  this 
instrument  was  L.  D.  Travis,  NASA  Goddard 
Institute  for  Space  Studies. 

The  photopolarimeter  (Figure  4-1)  weighed 
5  kg  (11  Ib)  and  required  5.4  W  of  electrical 
power.  It  consisted  of  a  3.7-cm  (1.5-in.) 
aperture  telescope  with  a  rotating  filter  wheel. 

There  were  16  active  positions  on  the  filter 
wheel,  three  filters  for  each  of  four  spectral 
bands  (255-285,  355-380,  540-555,  and  930- 
945  nm),  limb-scan  filters,  and  imaging  filters. 
A  Wollaston  prism  directed  the  light  beams  for 
the  photopolarimetry  channels  to  two  silicon 
photodiodes  enhanced  to  detect  ultraviolet 
light.  Diagonal  reflectors  at  two  positions  on 
the  back  of  the  filter  wheel  sent  the  beams  to 
two  other  silicon  photodiodes.  One  was  for  the 
imaging  channel  and  the  other  for  the 
limb-scan  channel. 

This  telescope  observed  Venus  at  fixed  angles. 
It  used  the  Orbiter's  rotation  to  lay  scans 
across  the  planet.  To  set  these  scans  side  by 
side,  it  used  the  motion  along  the  spacecraft's 
trajectory.  Ground  control  could  set  the  angle 
of  the  telescope's  axis  to  the  spacecraft's  spin 
axis.  By  this  means,  investigators  could  direct 
the  telescope  to  observe  the  planet  from  any 
point  along  the  Orbiter's  elliptical  orbit. 

In  the  imaging  mode  of  operation,  when  the 
spacecraft  measured  only  the  intensity  of 
received  radiation,  the  polarimeter's  field  of 
view  was  about  0.5  mrad.  This  corresponds  to 
a  resolution  of  about  30  km  (19  miles)  directly 
below  the  Orbiter.  In  this  mode,  approxi- 
mately 3.5  hours  were  required  to  record  an 
image  of  Venus'  full  disk.  An  ultraviolet  filter 
revealed  the  fast  moving  cloud  markings  that 
appear  only  in  ultraviolet  pictures  of  Venus.  A 
maximum  of  five  full-disk  planetary  images 
were  possible  during  each  spacecraft's  orbit. 

In  the  photopolarimetry  mode,  the  instrument's 
field  of  view  was  approximately  0.5°.  This 
corresponds  to  a  resolution  of  about  500  km 
(310  miles)  directly  below  the  Orbiter.  In  this 
mode,  the  photopolarimeter  used  four  pass- 
bands.  The  instrument  measured  polarization 
of  scattered  sunlight,  the  characteristics  of 
which  depend  on  particle  size,  shape,  and 

density  in  clouds  and  hazes.  These  data 
yielded  vertical  distribution  of  cloud  and  haze 
particles  relative  to  atmospheric  pressure. 

When  the  Orbiter  neared  periapsis,  the  instru- 
ment could  observe  in  visible  light  the  atmo- 
sphere's high  haze  layers.  This  was  done  by 
programming  the  telescope  to  scan  across  the 
limb  of  the  planet.  In  this  mode,  the  field  of 
view  was  about  0.25  mrad,  or  an  altitude  resolu- 
tion of  about  0.5  to  1.0  km  (0.3  to  0.6  miles). 
Such  observations  provided  information  about 
layers  above  Venus'  main  cloud  deck. 

In  Phase  II  of  the  Orbiter's  mission,  the 
instrument  provided  a  detailed  record  of  the 
long-term  evolution  of  significant  haze  effects. 
This  is  important  for  understanding  photo- 
chemical and  aerosol  processes  and  the 
atmosphere's  mechanisms  of  meridional 
transport.  From  observations  of  many  years 
during  Phase  II,  the  instrument  showed  build- 
up and  dissipation  of  midlatitude  jet  streams 
and  provided  insight  into  zonal  circulation. 

Surface  Radar  Mapper 
The  radar  mapping  instrument  (Figure  4-2) 
weighed  9.7  kg  (21.3  Ib)  and  required  18  W 
of  electrical  power.  The  radar  team  leader  was 
G.  H.  Pettengill,  Massachusetts  Institute  of 
Technology.  The  experiment  produced  the 
first  maps  of  large  areas  of  Venus  unobservable 
by  Earth-based  radar.  From  radar  echoes, 
experimenters  derived  surface  heights  along 
the  spacecraft's  suborbital  trajectory  to  an 
accuracy  of  150  m  (492  ft).  They  were  able  to 
make  a  good  estimate  of  global  topography 
and  shape.  The  team  also  derived  surface 
electrical  conductivity  and  meter-scale  rough- 
ness from  the  radar  data. 

A  low-power  (20  W  peak  pulse  power),  S-band 
(1.757  GHz)  radar  system  observed  the  surface. 
Ground  controllers  mechanically  moved  the 


Figure  4-2.  Orbiter  radar- 
mapping  instrument  (ORAD). 
(Left)  Electronics  box  compared 
to  15-cm  (6-in.)  scale.  (Right)  A 
38-cm  (15-in.)  diameter  short 
backfire  reflector  antenna  and  its 
supporting  structure  with  a  scale 
alongside  the  base. 


antenna  in  a  plane  containing  the  spacecraft's 
spin  axis.  The  controllers  did  this  to  view  the 
suborbital  point  on  the  planet's  surface  once 
during  each  spacecraft  roll.  The  Orbiter  took 
measurements  whenever  it  was  below  4700  km 
(2920  miles).  These  measurements  were  sub- 
ject to  constraints  that  the  spinning  spacecraft 
set.  They  also  had  to  compete  with  other  experi- 
ments for  the  Orbiter's  limited  telemetry  capac- 
ity. To  minimize  telemetry  requirements,  echoes 
were  processed  on  board  the  spacecraft.  The 
spacecraft  spun  at  a  rate  of  about  five  revolutions 
per  minute.  During  this  period,  radar  observa- 
tions occupied  about  1  second  out  of  the  total 
rotation  period  of  12  seconds.  The  instru- 
ment automatically  compensated  for  Doppler 
shift.  (Radial  motion  of  the  Orbiter  toward  and 
away  from  the  planet  during  each  elliptical 
orbit  caused  the  shift.)  When  the  spacecraft 
was  closer  than  700  km  (435  miles)  to  the 
surface,  the  received  frequency  was  stepped  to 
make  range  measurements  of  the  areas  lying 
just  ahead  and  behind  the  spacecraft's  path. 

Investigators  wanted  to  find  absolute  topo- 
graphical elevations.  To  do  this,  they  sub- 
tracted the  observed  distance  between  the 
Orbiter  and  the  surface  from  the  spacecraft's 
orbital  radius.  This  radius  was  obtained  from 

Deep  Space  Network  (DSN)  tracking  of  the 
spacecraft.  Surface  resolution  was  best  at 
periapsis.  It  was  then  23  km  (14  miles)  along 
the  track  and  7  km  (4.3  miles)  across  the  track. 
Relatively  long  pulses  were  used  to  obtain  a 
good  signal-to-noise  ratio  from  each  pulse. 

When  the  radar  operated  in  its  other  mode 
(namely,  side-looking  radar  imaging  at  alti- 
tudes below  550  km,  or  342  miles),  the  func- 
tional parameters  for  altimetry  measurements 
changed.  This  mode  relied  upon  uncoded 
pulses  at  a  pulse  repetition  frequency  of 
200  Hz  to  avoid  ambiguities  in  range  and 
surface  mapping.  The  antenna,  pointing  to 
one  or  both  sides  of  the  ground  track,  made  a 
sequence  of  surface  brightness  measurements. 
Commands  from  Earth  determined  which  way 
the  antenna  pointed.  The  illuminated  surface 
area  was  divided  into  64  picture  elements 
(pixels).  When  the  spacecraft  was  close  to 
periapsis,  each  pixel  covered  an  area  about 
23  km  (14.3  miles)  square  on  the  surface. 

The  radar  mapper  operated  during  Phase  I  and 
part  of  Phase  II  of  the  mission  when  the  peri- 
apsis altitude  was  low  enough  for  useful  radar 
images.  This  period  of  low  periapsis  ended 
March  31,  1981,  and  the  mapper  experiment 

Figure  4-3.  Orbiter  infrared 
radiometer  (OIR).  (Top)  Cutaway 
drawing  of  instrument  related 
to  outline  of  its  housing.  (Bottom 
left)  Packaged  instrument. 
(Bottom  right)  Instrument  with- 
out its  housing. 

ended.  Toward  the  end  of  Phase  III  of  the  mis- 
sion, the  periapsis  altitude  was  again  suitable 
for  radar  mapping.  However,  because  the  Magel- 
lan spacecraft  was  by  that  time  producing  higher 
resolution  radar  imaging,  project  management 
did  not  reactivate  the  Orbiter's  instrument. 

Infrared  Radiometer 

The  infrared  radiometer  (Figure  4-3)  weighed 
5.9  kg  (13  Ib)  and  required  5.2  W  of  electrical 
power.  The  principal  investigator  was  F.  W. 
Taylor,  Oxford  University,  England.  The  uni- 
versity developed  and  constructed  a  pressure 
modulation  unit  and  molecular  sieve  for  one 
channel  of  the  instrument  to  make  measure- 

ments over  a  wide  range  of  temperatures  and 
pressures.  The  radiometer  measured  infrared 
radiation  that  Venus'  atmosphere  emitted  at 
various  altitudes.  These  altitudes  ranged  from 
60  km  (37  miles)  at  the  top  of  the  cloud  deck, 
where  the  atmospheric  pressure  was  250  mbars, 
to  150  km  (93  miles),  where  the  pressure  was 
10-6  mbars.  This  region  includes  those  parts  of 
Venus'  atmosphere  where  the  four-day  circula- 
tion takes  place,  where  there  is  maximum 
cooling  by  radiation,  and  where  there  is  maxi- 
mum deposition  of  solar  energy.  The  instru- 
ment searched  for  water  vapor  above  the  cloud 
layers  and  measured  the  extent  of  the  heat- 
trapping  cloud  layers,  and  measured  the 



albedo.  Data  from  the  radiometer  yielded 
about  800,000  vertical  profiles  of  upper  atmo- 
sphere temperatures.  By  keeping  sample  time 
short,  scientists  were  able  to  obtain  a  tempera- 
ture sensitivity  of  more  than  0.5  K  at  240  K. 
Such  information  was  important  for  discover- 
ing both  the  extent  and  the  driving  forces  of 
the  upper  atmosphere's  four-day  circulation. 

The  radiometer  had  eight  detectors,  each 
sensitive  to  a  different  part  of  the  spectrum. 
Because  the  instrument  covered  such  a  wide 
spectrum  range,  it  needed  several  different 
measurement  techniques.  Five  detectors 
measured  infrared  emissions  at  five  selected 
wavelengths  of  the  absorption  band  for  carbon 
dioxide  near  15  microns.  Each  wavelength 
sampled  a  specific  altitude  region  in  the 
atmosphere,  depending  on  the  heat-absorbing 
traits  of  the  carbon  dioxide  molecule  and  the 
temperature  variation  with  altitude.  One 
detector  exclusively  detected  and  mapped 
water  vapor  distribution  in  the  upper  atmo- 
sphere. This  device  centered  on  the  strongest 
part  of  the  pure  rotational  band  of  water  vapor 
at  40  to  50  microns.  Another  instrument, 
operating  in  the  2.0-micron  band  of  carbon 
dioxide,  measured  cloud  layer  size  and  shape. 
The  wide-band  albedo  channel  from  0.2  to 
4.5  microns  measured  total  solar  reflectance. 

A  48-mm  (1.9-in.)  aperture  parabolic  mirror 
gathered  radiation  for  all  eight  channels  of 
the  instrument.  The  instrument's  axis  was  at 
45°  to  the  Orbiter's  spin  axis.  This  position 
allowed  rotation  of  the  spacecraft  to  scan  the 
instrument's  field  of  view  across  the  planet. 
When  looking  at  the  limb  of  the  planet,  the 
instrument  provided  a  vertical  resolution  of 
5  km  (3  miles)  at  periapsis. 

Unfortunately,  on  February  4,  1979,  the 
radiometer  malfunctioned  after  72  orbits,  and 
could  no  longer  be  operated. 

Airglow  Ultraviolet  Spectrometer 
The  airglow  ultraviolet  spectrometer  mapped 
and  made  spectroscopic  analyses  of  ultraviolet 
light  that  Venus'  clouds  and  gases  scattered  or 
emitted.  The  instrument  (Figure  4-4)  weighed 
3.1  kg  (6.8  Ib)  and  required  1.7  W  of  electrical 
power.  The  principal  investigator  was  A.  I. 
Stewart,  University  of  Colorado. 

How  the  planet's  clouds  and  atmosphere 
reflect  ultraviolet  sunlight  depends  on  the 
details  of  the  size  and  makeup  of  cloud  aerosols. 
It  also  depends  on  distribution  of  ultraviolet- 
absorbing  gases.  Both  spectral  intensity  (how 
the  brightness  of  the  light  varies  with  its 
wavelength)  and  maps,  or  images,  carry  the 
"finger-print"  of  these  factors.  Analysis  of  such 
information  reveals  three-dimensional  details 
of  the  distribution  of  clouds,  hazes,  and  gases. 
From  images  they  made  on  successive  days, 
investigators  traced  the  variations  and  move- 
ment of  gas  bodies  and  cloud  markings  that 
can  be  seen  only  in  ultraviolet  light. 

Absorption  of  extreme  ultraviolet  radiation 
from  the  Sun  by  the  upper  atmosphere's  gases 
causes  a  fluorescence  known  as  "airglow."  Each 
gas  has  its  own  special  emissions,  and  each  of 
the  many  physical  and  chemical  processes 
involved  in  airglow  has  its  own  characteristics, 
too.  By  measuring  emissions,  experimenters 
sought  to  learn  how  the  Sun's  radiation 
modifies  the  upper  atmosphere's  composition 
and  temperature. 

One  of  Venus'  big  mysteries  is  why  it  lacks 
water.  The  ultraviolet  spectrometer  helped 
solve  this  problem.  It  measured  the  emission 
of  Lyman-alpha  radiation  from  hydrogen 
atoms  that  form  a  corona  around  Venus. 
Scientists  used  this  measurement  to  derive  the 
amount  of  hydrogen  that  must  be  escaping 
from  the  top  of  the  planet's  atmosphere.  The 
information  is  important  because  escaping 


Logic  assembly 


Mounting  wedge 


Figure  4-4.  Orbiter  ultraviolet 
spectrometer  (OUVS).  The 
diagram  identifies  the  instru- 
ment's major  components. 

atomic  hydrogen  is  the  last  step  before  a 
planet  loses  water.  Incoming  solar  radiation 
breaks  water  into  hydrogen  and  oxygen  by 
photolytic  processes.  The  oxygen  is  too  heavy 
to  escape  from  a  planet  the  size  of  Venus,  but 
hydrogen  can  escape  into  space  from  the  top 
of  the  atmosphere.  Yet,  this  process  does  not 
account  for  the  extreme  dryness  of  Venus 
today.  Scientists  needed  much  more  informa- 
tion about  conditions  on  Venus. 

The  spectrometer  featured  a  5-cm  (2-in.) 
aperture  f/5  Cassegrain  telescope,  protected  by 
a  light  shade.  It  had  an  f/5,  12.5-cm  (5-in.) 
focal  length  monochromator  of  Ebert-Fastie 
design.  The  monochromator  used  a  diffraction 
grating  with  3600  grooves  per  millimeter.  A 

programmable  step  motor  commanded  from 
Earth  was  used  to  select  the  desired  wave- 
length for  each  observation.  The  spectral 
resolution  was  13  angstroms,  and  each  grating 
step  was  4.4  angstroms.  (An  angstrom  is  a  unit 
of  wavelength  equal  to  10-8  cm;  this  is  approx- 
imately the  diameter  of  a  hydrogen  atom.) 
Two  exit  slits  passed  the  dispersed  light  from 
the  monochromator  to  two  photomultiplier 
tubes.  They  converted  the  light  from  Venus 
into  electrical  impulses  that  the  spacecraft 
then  telemetered  back  to  Earth. 

One  photomultiplier  had  a  cesium  iodide 
cathode  with  a  lithium  fluoride  window.  It  was 
sensitive  to  the  wavelength  range  from  1100  to 
1900  angstroms.  The  other  had  a  cesium 



telluride  cathode  and  a  quartz  window  and 
was  sensitive  from  1800  to  3400  angstroms. 

The  instrument  could  operate  in  several 
modes.  In  the  spectral  mode,  it  scanned  the 
complete  spectrum  in  four  256-word  sections. 
Each  section  was  acquired  in  1  second  and 
required  one  or  more  complete  spins  of  the 
spacecraft  to  transmit  it  to  Earth. 

The  spectrometer  performed  mapping  and 
imaging  in  the  wavelength  mode.  In  that  mode, 
commands  from  Earth  selected  the  grating 
position  to  choose  the  wavelength,  the  detec- 
tor tube,  and  the  length  and  location  of  the 
data  arc.  If  the  instrument  command  system 
or  data  memory  failed,  backup  modes  with 
lesser  capabilities  were  available  to  ensure 
data  collection. 

On  a  typical  orbit,  the  ultraviolet  spectrometer 
viewed  the  planet  from  150  to  35  minutes 
before  periapsis.  It  viewed  the  planet  again 
15  minutes  before  periapsis  to  10  minutes 
after.  The  first  period  gathered  airglow  and 
cloud  images.  The  second  obtained  data  for 
studying  limb  airglow  profiles  and  limb  hazes. 
For  the  rest  of  the  orbit,  the  instrument 
observed  bright,  hot  stars  for  calibration  pur- 
poses. Measurements  could  be  made  of 
Lyman-alpha  radiation  emitted  by  hydrogen 
atoms  throughout  the  orbit. 

The  principal  investigator's  objectives  for  the 
extended  mission  included  continued  mea- 
surements of  Venus  and  measurements  of 
selected  comets.  The  spectrometer  mapped 
and  monitored  the  distribution  of  two  compo- 
nents of  the  dayside  Venusian  thermosphere. 
One  was  the  horizontal  distribution  of  atomic 
oxygen.  The  other  was  the  horizontal  and 
vertical  distribution  of  carbon  monoxide.  The 

aim  was  to  characterize  circulation  properties 
and  the  role  of  vertical  eddy  mixing  in  this 
region.  The  instrument  also  determined 
dependence  on  solar  activity  of  the  dayside 
and  nightside  circulation  patterns  within  the 
thermosphere.  Another  aim  was  to  determine 
long-term  behavior  of  sulfur  dioxide  in  the 
cloud  tops.  Additionally,  Phase  II  operations 
showed  how  Venus'  hydrogen  corona  responded 
to  changes  in  solar  activity  during  an  entire 
solar  cycle. 

Comets  were  a  target  of  opportunity  for  this 
instrument.  As  a  result,  it  had  a  number  of 
research  objectives  that  were  finding  out  how 
comets  lose  water,  the  ratios  of  carbon, 
oxygen,  and  hydrogen,  the  rotation  rate  of  the 
nucleus,  and  the  extent  and  nature  of  the 
ultraviolet  emitting  coma. 

Neutral  Mass  Spectrometer 
The  neutral  mass  spectrometer  (Figure  4-5)  was 
one  of  two  mass  spectrometers  that  the  Orbiter 
carried.  It  weighed  3.8  kg  (8.4  Ib)  and  required 
an  average  of  12  W  of  electrical  power.  It  mea- 
sured the  densities  of  neutral  atoms  and  mole- 
cules in  an  upper  atmosphere  range.  That 
range  extended  from  near  periapsis  to  a  maxi- 
mum altitude  of  500  km  (311  miles).  Principal 
investigator  was  H.  B.  Niemann,  NASA 
Goddard  Space  Flight  Center.  Information 
about  vertical  and  horizontal  distributions  of 
neutral  gas  molecules  was  important.  Scientists 
could  use  it  to  define  the  chemical,  dynamical, 
and  thermal  state  of  Venus'  upper  atmosphere. 
Researchers  also  were  able  to  determine  the 
height  above  the  planet's  surface  at  which  atmo- 
spheric mixing  ends.  (This  region  is  the  turbo- 
pause.)  They  did  this  by  comparing  inert  gas 
densities  at  altitudes  accessible  to  Orbiter  with 
densities  that  the  Large  Probe  and  the  Multi- 
probe  Bus  measured  below  150  km  (93  miles). 

Entrance  aperture 

Electron  beam 

Electron  gun 


Retarding  grid 

Ion  repeller 
Focus  grid 
Ion  lens 


Ion  detector 





Getter  pump 



quadrupole  rods 


Quadrupole  mass 










Figure  4-5,  Orbiter  neutral 
mass  spectrometer  (ONMS). 
(Top  left)  A  simplified  diagram 
of  the  instrument  with  entrance 
orifice  at  top.  (Top  right)  Photo- 
graph of  assembled  instrument. 
(Middle)  A  more  detailed 
diagram  showing  whole  instru- 
ment in  longitudinal  cross 
section.  (Bottom)  A  cutaway 
perspective  of  the  instrument 
with  spectrometer  separated 
from  its  housing.  The  breakoff 
cap  covering  the  inlet  during 
cruise  to  Venus  is  to  the  left  of 
this  drawing. 



Investigators  identified  and  measured  noble 
gases,  other  nonreactive  gases,  and  chemically 
active  gases  of  up  to  46  atomic  mass  units. 
They  used  a  quadrapole  mass  spectrometer 
with  an  electron-impact  ion  source  and  a 
secondary  electron  multiplier  ion  detector.  The 
instrument  first  ionized  gas  molecules.  Then  a 
quadrapole  mass  filter  separated  them  accord- 
ing to  their  mass.  The  ion  source  was  inside  a 
chamber  that  connected  to  the  outside  atmo- 
sphere via  a  knife-edged  orifice.  It  operated  in 
two  modes  alternately:  an  open-source  mode 
and  a  closed-source  mode. 

In  the  open-source  mode,  the  device  analyzed 
only  those  ions  that  came  from  ionization  of 
free-streaming  particles.  Such  particles  had  a 
large  kinetic  energy  with  respect  to  the  Orbiter 
since  it  was  moving  through  the  atmosphere 
at  nearly  10  km/sec  (6.2  miles/sec)  at  periapsis. 
For  atomic  oxygen,  this  kinetic  energy  was 
about  8  eV.  By  contrast,  it  was  about  0.025  eV 
for  surface-reflected  particles.  A  retarding 
potential  analysis  discriminated  between 
surface-reflected  and  free-streaming  particles 
after  the  electron  beam  had  ionized  them.  To 
be  effective  near  periapsis,  the  mass  spectrom- 
eter's axis  had  to  point  in  the  general  direction 
of  the  Orbiter's  motion  once  per  spin  period. 
Researchers  accomplished  this  by  mounting 
the  device  on  the  spacecraft's  instrument  plat- 
form so  its  axis  was  27°  from  the  spacecraft's 
spin  axis.  In  this  mode,  the  instrument 
measured  concentrations  of  chemically  active 
gases,  such  as  atomic  oxygen. 

In  the  closed-source  mode  of  operation,  almost 
all  particles  the  instrument  analyzed  were 
surface-reflected  particles.  The  gas  density  in 
the  ion  source  was  significantly  enhanced 
because  inflowing  gas  stagnated  in  the  source 
chamber.  This  mode  was  suitable  for  deter- 
mining concentrations  of  noble  gases,  such 
as  helium,  and  of  nonreactive  gases,  such  as 

carbon  dioxide  and  molecular  nitrogen. 
Surface-reflected  particles  adjusted  to  the  sur- 
face temperature  before  making  multiple  passes 
through  the  ionization  region.  As  a  result,  this 
mode  had  enhanced  sensitivity.  This  permitted 
measurements  to  much  lower  concentrations 
than  was  possible  in  the  open-source  mode. 

To  keep  internal  surfaces  clean  and  allow 
instrument  testing  during  launch  preparations 
and  cruise,  a  metal-ceramic  breakoff  cap 
covered  the  ion  source.  It  maintained  the 
internal  pressure  below  10-4  Pa  (10-6  torr).  A 
pyrotechnic  actuator  removed  the  cap  after 
the  spacecraft  entered  orbit. 

Ground  commands  could  program  the  mass 
spectrometer  to  scan  continuously  from  1  to 
46  atomic  mass  units,  or  to  scan  any  combina- 
tion of  eight  masses  within  that  range.  The 
kinetic  energy  of  the  ionizing  electrons  could 
be  chosen  by  ground  command  to  be  70  or 
27  eV,  so  constituents  of  equal  mass  could  be 
discriminated  during  analysis. 

In  Phase  I,  the  instrument  made  measure- 
ments within  the  neutral  atmosphere  below 
250  km  (155  miles)  in  the  planet's  northern 
hemisphere.  In  Phase  III,  the  orbit  was  ori- 
ented so  measurements  could  be  made  in  the 
southern  hemisphere. 

Solar  Wind  Plasma  Analyzer 
The  solar-wind  plasma  analyzer  (Figure  4-6) 
weighed  3.9  kg  (8.6  Ib)  and  required  5  W  of 
electrical  power.  It  measured  the  velocity, 
density,  flow  direction,  and  temperature  of  the 
solar  wind,  and  its  interactions  with  Venus' 
ionosphere  and  upper  atmosphere.  Principal 
investigator  for  the  solar-wind  plasma  experi- 
ment was  initially].  H.  Wolfe,  NASA  Ames 
Research  Center.  A.  Barnes,  also  of  Ames 
Research  Center,  succeeded  him. 

The  plasma  analyzer  was  an  electrostatic, 
energy-per-unit-charge  spectrometer.  Mounted 
near  the  outer  edge  of  the  equipment  shelf,  the 
instrument  had  a  field  of  view  normal  to  the 
spacecraft's  spin  axis.  The  field  of  view  rotated 
with  the  spacecraft.  The  rate  of  flow  (flux)  of 
the  solar  wind  was  measured  by  the  deflection 
of  incoming  particles  subjected  to  an  electro- 
static field  between  two  metal  plates.  If  the 
particles  were  within  the  range  of  energy  and 
incidence  determined  by  the  aperture's  orien- 
tation and  the  voltage  between  the  plates,  they 
exited  to  hit  one  of  five  detectors.  Which  target 
detector  a  solar-wind  particle  hit  depended  on 
the  wind's  direction.  By  varying  the  voltage 
between  plates,  scientists  could  measure  a  com- 
plete solar-wind  particle  velocity  distribution. 

The  instrument's  analyzer  section  was  a  nested 
pair  of  quadrispherical  plates  with  a  mean 
radius  of  12  cm  (4.72  in.).  These  plates  were 
1.0  cm  (0.39  in.)  apart.  Charged  particles,  such 
as  protons  and  electrons,  that  passed  through 
the  instrument's  entrance  aperture  entered  the 
region  between  the  charged  plates.  There  the 
electrostatic  field  deflected  them  into  a  curved 
path.  Following  this,  an  array  of  five  current 
collectors — located  at  the  curved  plate's  exit 
end — collected  them.  Each  target  was  con- 
nected to  an  electrometer  amplifier. 

The  instrument  had  two  modes  of  operation 
that  scientists  could  command  from  Earth:  a 
scan  mode  and  a  step  mode. 

The  scan  mode  first  found  the  maximum  flux 
over  one  spacecraft  rotation  for  each  voltage 
step.  It  then  identified  the  collector  and  space- 
craft azimuth  of  this  maximum  flow.  The 
energy/charge  range  was  normally  32  loga- 
rithmically equal  steps  over  the  range  of  50 
to  8000  V  for  high-energy  positive  ions,  or 
15  steps  from  3  to  250  V  plus  a  zero  step  at 
0.25  V  for  electrons  and  low-energy  positive 

Drift  tube 

Suppressor  grid 
Target  ground 

Analyzer  plates 

Drift  tube 



Figure  4-6.  Orbiter 
solar-wind  plasma 
analyzer  (OPA).  (Top) 
Diagram  showing 
arrangement  of  the 
curved,  electrostati- 
cally charged  plates 
with  respect  to  the 
five  detectors  that 
recorded  velocity  of 
solar-wind  particles. 
(Bottom)  Photograph 
of  assembled  instru- 
ment in  its  housing. 



ions.  Next,  the  instrument  made  a  polar  and  an 
azimuthal  scan.  It  did  this  at  the  four  consecu- 
tive steps  beginning  with  the  step  before  the 
one  in  which  it  measured  the  peak  flux.  Each 
polar  scan  measured  the  flux  at  all  five 
collectors  at  each  step.  All  azimuth  scans  mea- 
sured the  flux  in  12  sectors  centered  on  the 
peak  flux  direction. 

In  the  step  mode,  only  maximum  flux  scan 
occurred,  with  about  1  second  allocated  to 
each  voltage. 

During  the  early  part  of  Phase  II,  this  experi- 
ment increased  our  understanding  of  condi- 
tions within  the  ionosheath.  A  more  detailed 
knowledge  of  bow  shock  allowed  calculation 
of  how  much  solar  wind  the  planet's  iono- 
sphere absorbs.  In  Phase  III,  the  instrument 
gathered  data  similar  to  that  in  Phase  I  but  at 
a  different  part  of  the  solar  cycle. 


A  flux-gate  magnetometer  recorded  Venus' 
extremely  weak  magnetic  field.  The  instrument 
weighed  2  kg  (4.44  Ib)  and  required  2.2  W  of 
electrical  power.  The  principal  investigator  was 
C.  T.  Russell,  University  of  California, 
Los  Angeles.  The  magnetometer  searched  for 
surface-correlated  magnetic  features.  These 
included  regions  of  Venusian  crust  that  might 
have  been  magnetized  in  the  past.  If  present, 
the  features  would  have  shown  that  Venus 
once  had  a  field  more  like  Earth's.  Although 
Venus'  magnetic  field  is  extremely  weak,  scien- 
tists thought  that  it  might  play  an  important 
part  in  the  interactions  between  solar  wind 
and  the  planet.  Their  aim  was  to  clarify  whether 
solar  wind  was  deflected  by  a  field  intrinsic  to 
Venus,  by  an  induced  field,  or  by  the 
ionosphere  itself. 

The  magnetometer  consisted  of  three  sensors 
mounted  on  a  4.7-m  (15.4-ft)  boom.  The  long 
boom  isolated  the  sensors  from  the  spacecraft's 
magnetic  field.  This  feature  allowed  it  to 
measure  weak  fields  in  the  nanotesla  (nT)  or 
gamma  range.  (The  field  of  Earth  at  its  surface 
is  about  50,000  nT.)  Two  sensors  were  at  the 
end  of  the  boom:  one  parallel  to  the  space- 
craft's spin  axis  and  the  other  perpendicular  to 
it.  An  inboard  sensor,  one-third  of  the  way 
down  the  boom,  tilted  45°  to  the  spin  axis. 
This  inner  sensor  measured  the  Orbiter's 
magnetic  field.  This  value  was  subtracted  from 
the  readings  of  the  outboard  sensors  to  correct 
them  for  the  spacecraft's  presence.  Each  sensor 
consisted  of  a  ring,  around  which  was  wrapped 
a  ribbon  of  permeable  metal  to  form  the 
sensor's  core.  It  was  surrounded  with  drive, 
sense,  and  feedback  coils.  Any  external  field 
caused  the  core  to  produce  an  electrical  signal. 
A  feedback  signal  then  canceled  the  external 
field  so  the  magnetometer  always  operated  in 
a  zero-field  condition.  The  strength  of  the 
feedback  signal  needed  to  produce  the  zero- 
field  condition  was  a  measure  of  the  external 
magnetic  field. 

Engineers  designed  the  magnetometer  so  that 
it  did  not  need  gain  changes  when  it  moved 
to  and  from  low-  or  high-field  regions.  The 
instrument's  range  remained  fixed  at  128  nT. 
The  resolution,  however,  changed  from 
0.0625  nT  to  plus  or  minus  0.5  nT  in  response 
to  field  changes. 

During  Phase  II,  investigators  had  a  number  of 
objectives.  Among  them  were  gathering  new 
information  about  solar-wind  interaction  with 
the  equatorial  ionosphere  and  determining 
how  much  material  is  lost  into  space  from 
Venus'  atmosphere.  They  also  were  interested 
in  learning  how  energy  moves  from  solar  wind 
to  ionosphere  and  about  conditions  in  Venus' 
wake  and  tail.  Phase  II  repeated  the  geometry 

Edge  of 



Inside  surface 

of  launch 

vehicle  fairing 

of  Phase  I  but  made  measurements  in  a  differ- 
ent part  of  the  solar  cycle.  Further,  at  the  end 
of  Phase  III,  the  Orbiter  carried  the  magnetom- 
eter rapidly  through  the  ionosphere.  This 
allowed  the  instrument  to  obtain  data,  at  least 
for  several  periapsis  passages,  at  altitudes  lower 
than  in  Phase  I. 

Electric  Field  Detector 
Investigators  designed  the  electric  field 
detector  (Figure  4-7)  to  answer  questions  about 
interactions  between  Venus  and  the  solar 
wind.  The  instrument  weighed  0.8  kg  (1.76  Ib) 
and  required  0.7  W  of  electrical  power.  The 
principal  investigator  was  initially  F.  L.  Scarf, 
TRW  Systems.  R.  J.  Strangeway,  University  of 
California  at  Los  Angeles,  later  replaced  him. 
The  instrument  provided  information  about 
how  Venus  deflected  solar  wind  around  the 
planet  and  how  much  the  solar  wind  heated 
the  ionosphere.  It  also  provided  data  about  the 
extent  of  ionization  that  the  exosphere-solar- 
wind  interaction  caused.  It  gave  information 
about  solar-wind  turbulence,  too.  Additionally, 
it  allowed  scientists  to  measure  variable 

locations  of  the  bow  shock,  the  ionopause, 
and  the  wake-cavity  boundary. 

The  electric  field  detector  measured  electric 
components  of  plasma  waves  and  radio 
emissions  in  the  frequency  region  from  50  to 
50,000  Hz.  Currents  were  induced  in  a  66-cm 
(26-in.)  long  V-type  electric  dipole  antenna, 
and  they  were  amplified  to  relay  information 
to  Earth.  Four  30%  bandwidth  channels, 
centered  at  100,  730,  5400,  and  30,000  Hz, 
were  used.  Each  was  needed  at  different  points 
along  the  spacecraft's  orbit  when  the  Orbiter 
passed  through  varying  densities  of  solar  wind. 

The  instrument  also  searched  for  "whistlers," 
or  electromagnetic  disturbances  that  travel 
along  a  magnetic  field  line.  Scientists  designed 
the  device  so  that  it  could  detect  electron 
whistler  mode  signals  in  the  100-Hz  channel  at 
all  orbital  locations. 

Observations  during  Phase  II  paid  valuable 
dividends.  They  provided  an  extended  data- 
base to  assess  time  variations  and  solar  cycle 

Figure  4-7.  Orbiter  electric  field 
detector  (OEFD).  (Left)  Photo- 
graph of  V-type  antenna  and 
its  mounting.  (Right)  Diagram 
to  illustrate  how  antenna  was 
stowed  within  the  launch 
vehicle  and  antenna's  position 
when  deployed. 


Figure  4-8.  Orbiter  electron 
temperature  probe  (OETP). 
Two  Langmuir  probes  mounted 
outside  the  spacecraft  and  the 
electronics  package. 


effects  on  the  instrument's  measurements,  and 
their  implications. 

Electron  Temperature  Probe 
The  electron  temperature  probe  measured 
thermal  properties  of  Venus'  ionosphere.  Mea- 
surements included  electron  temperature,  elec- 
tron concentration,  ion  concentration,  and  the 
spacecraft's  own  electrical  potential.  Scientists 
needed  such  measurements  to  help  them  under- 
stand how  the  ionosphere  obtains  heat.  The 
principal  investigator  for  the  electron  tempera- 
ture experiment  was  L.  H.  Brace,  NASA 
Goddard  Space  Flight  Center. 

The  probe  (Figure  4-8)  weighed  2.2  kg  (4.76  Ib) 
and  required  4.8  W  of  electrical  power.  It  con- 
sisted of  two  cylindrical  Langmuir  probes:  an 
axial  probe  and  a  radial  probe.  The  former  was 
mounted  parallel  to  the  spacecraft's  spin  axis 
at  the  end  of  a  boom  that  was  40  cm  (15.75  in.) 
long.  The  latter  was  mounted  at  the  end  of  a 
1-m  (39.37-in.)  boom  that  extended  radially 
from  the  spacecraft's  periphery.  Each  probe 
was  7  cm  (2.8  in.)  long  and  0.25  cm  (0.1  in.)  in 
diameter.  Both  probes  had  their  own  power 
generator  but  shared  in-flight  data 
analysis  circuitry. 

A  sawtooth  voltage  swept  each  probe  twice  a 
second.  The  voltage  was  electronically  adapted 
to  match  the  existing  electron  density  and 
temperature  being  measured.  The  sweep 
amplitude  varied  automatically  over  the  range 
0.5  to  10  V,  to  suit  the  electron  temperature 
being  measured.  Appropriate  bias  voltages 
were  added  to  compensate  for  the  spacecraft's 
potential.  At  the  beginning  of  each  sweep, 
automatic  current-ranging  circuits  sampled  this 
ion  current.  They  adjusted  the  electrometer 
gain  to  suit  the  variations  in  ion  concentra- 
tion. The  instrument's  design  included  such 
adaptive  functions  so  the  resolution  could  be 
as  large  as  possible  over  a  wide  range  of 
electron  concentrations  and  temperatures. 

A  commandable  mode  permitted  sampling  of 
one  probe  instead  of  alternating  between  two 
probes.  This  allowed  experimenters  to  take 
advantage  of  having  two  probes  that,  because 
of  their  orientation,  responded  differently  to 
changes  in  electron  concentration  while 
maintaining  high  spatial  resolution. 

During  Phase  II,  this  instrument  investigated 
two  important  regions  of  Venus'  ionosphere. 
The  first  included  the  ionopause,  ionosheath, 
and  bow  shock  in  the  front  stagnation  region. 


The  second  included  ionosphere,  ionopause, 
and  wake  in  the  region  immediately  downwind 
from  Venus.  In  Phase  III,  the  southward  drift  of 
periapsis  allowed  the  spacecraft  to  examine 
different  ionospheric  regions. 

Ion  Mass  Spectrometer 
The  ion  mass  spectrometer  (Figure  4-9) 
weighed  3  kg  (6.6  Ib)  and  required  1.5  W  of 
electrical  power.  It  measured  the  distribution 
and  concentration  of  positively  charged  ions 
in  Venus'  atmosphere  above  150  km  (93  miles). 
The  spectrometer  was  similar  to  the  instrument 
the  Multiprobe  Bus  carried.  The  principal 
investigator  for  both  ion  mass  spectrometers 
was  initially  H.  A.  Taylor,  Jr.,  NASA  Goddard 
Space  Flight  Center  (1974-1988).  P.  R.  Cloutier, 
Rice  University,  replaced  him.  The  instrument 
directly  measured  ions  in  a  mass  range  from 
1  (protons  or  hydrogen  ions)  to  56  atomic  mass 
units.  Scientists  wanted  the  data  for  a  greater 
understanding  of  Venus'  ionosphere  and  its 
solar-wind  interactions. 

The  basic  measurement  cycle  was  6.3  seconds. 
The  instrument  first  made  an  exploratory 
sweep  of  1.8  seconds.  This  explore  mode 
searched  for  up  to  16  different  ions.  Then  the 
instrument  entered  its  adapt  mode  and  made  a 
series  of  sweeps  for  4.5  seconds.  The  device 
repeated  the  sampling  of  the  eight  most 
prominent  ions  that  it  identified  during  the 
exploratory  sweep.  The  instrument  the  Orbiter 
used  had  commandable  modes  to  regulate  its 
explore-adapt  logic  circuit.  This  allowed  the 
number  of  prominent  ions  for  adaptive  repeats 
to  be  reduced  from  8  to  4  or  2.  A  commandable 
option  also  allowed  the  spectrometer  to  remain 
in  the  explore  mode. 

In  flight,  the  sensor — a  Bennett-type  radio- 
frequency  ion  mass  spectrometer  tube — 
encountered  a  stream  of  atmospheric  ions. 
They  flowed  into  an  aluminum  cylinder 

enclosing  a  series  of  parallel  wire  grids.  Next,  a 
variable  negative  sweep  potential  accelerated 
each  ion  species  along  the  spectrometer's  axis. 
Engineers  programmed  this  process  to  step  and 
then  dwell  at  voltage  levels  needed  to  detect 
particular  ions.  In  this  way,  ions  that  passed 
through  the  radio-frequency  analyzer  stages 
in  phase  with  the  applied  voltage  gained 
sufficient  energy  to  penetrate  a  retarding 
direct-current  field  and  impinge  on  a  collector 
at  the  rear  of  the  sensor  cylinder.  The  ion 
stream's  accelerating  voltage  yielded  the  iden- 
tity of  the  ions  and  its  amplitude  revealed  their 
concentration.  A  dual  collector  system  that 
consisted  of  a  low-gain  grid  collector  and  a  high- 
gain  solid  disk  collector  detected  ion  currents. 

In  Phase  II  and  Phase  III,  the  instrument 
investigated  the  superthermal  ion  concentra- 
tions and  flow  properties  in  the  upper  altitude 
regions  of  Venus'  wake.  It  also  gathered  data 
about  structural  details  of  superthermal  ion 
distribution  in  the  ionopause,  ionosheath,  and 
bow  shock  regions. 

Charged-Particle  Retarding  Potential  Analyzer 
The  charged-particle  retarding  potential 
analyzer  measured  temperature,  concentra- 
tions, and  velocity  of  the  most  abundant  ions 
in  the  ionosphere.  It  also  measured  concentra- 
tion and  energy  distribution  of  photoelectrons 
in  the  ionosphere,  temperature  of  thermal 
electrons,  and  the  spacecraft's  potential. 
The  analyzer  provided  experimenters  with 
important  data  on  plasma  quantities  in  the 
ionosphere,  planetary  tail,  and  boundary 
layers  surrounding  Venus. 

The  instrument  weighed  2.8  kg  (6.2  Ib)  and 
required  2.4  W  of  electrical  power.  The 
principal  investigator  was  W.  C.  Knudsen, 
initially  with  Lockheed  Missiles  and  Space 
Company  and  later  with  Knudsen  Research. 
The  Fraunhofer  Institut  fur  Physikalische 


Figure  4-9.  Orbiter  and  Multi- 
probe  Bus  ion  mass  spectro- 
meter (OIMS/BIMS).  (Top) 
Schematic  diagram  to  show 
sensor  components  and  equa- 
tions used  to  derive  results. 
(Bottom)  Photograph  of 
assembled  instrument  with 
inlet  to  the  left. 


positive  ions 

(1)  Sensor  at  rest  relative  to  plasma 

M  = 

.    K|Va| 


(2)  Sensor  moving  relative  to  plasma 
K(|Va|-1/2mv2  +  <j>sc) 

M  = 



M  =  Mass  of  ion  (amu) 

Va  =  Accelerating  voltage 

m  =  Mass  of  ion 

v  =  Sum  of  spacecraft  and 
ion  velocities 

<|)sc  =  Spacecraft  charge 

=  Inter-grid  spacing 

F  =  rf  (radio  frequency) 

K  =  Constant 

Guard  ring 



(-«-  1.9  cm  —I 
GO  G-|  G2  G3  G4 

Figure  4-10.  Orbiter  charged 
particle  retarding  potential 
analyzer  (ORPA).  (Left)  Simpli- 
fied diagram  to  illustrate 
arrangement  of  grid  system 
and  detectors.  (Right)  Photo- 
graph of  complete  instrument 
with  circular  ground  plane  at 
top.  The  scale  at  bottom  of  the 
instrument  is  15  cm  (6  in.). 

Weltraumforschung,  West  Germany,  devel- 
oped and  fabricated  the  instrument's  sensor. 

The  instrument  (Figure  4-10)  detected  low- 
energy  plasma  particles  in  Venus'  ionosphere, 
as  opposed  to  the  much  more  highly  energized 
solar-wind  particles.  Nevertheless,  the  analyzer 
did  provide  data  about  the  interaction  between 
ionosphere  and  solar  wind  at  an  altitude  of  400 
to  500  km  (249  to  311  miles).  This  is  the  level 
where  solar-wind  streams  into  the  ionosphere. 

Because  of  their  varying  electrical  potentials, 
6-cm  (2.4-in.)  diameter  collector  grids  selec- 
tively allowed  various  ionospheric  particles  to 
strike  a  detector.  An  electrometer-amplified 
current  was  induced  in  the  detector.  Large 
entrance  grids  and  a  collector  guard  ring  pro- 
vided a  uniform  flux  radially  from  the  instru- 
ment's axis.  The  collector  sampled  the  central 
region  of  this  flux.  Multiple  retarding  grids, 
coated  with  colloidal  graphite,  kept  systematic 
error  low.  Surrounding  the  entrance  grid  was  a 
30-cm  (11.8-in.)  diameter  ground  plane.  This 
ensured  that  the  plasma  sheath  remained 
planar  even  at  low  electron  concentrations. 

By  applying  control  voltages  and  a  special 
program,  the  investigator  could  operate  the 
instrument  in  three  modes.  These  modes  were 
an  electron  Langmuir  probe  mode,  an  ion 
mode,  and  a  photoelectron  mode.  Onboard 
data  analysis  by  the  instrument  selected  the 
optimum  point  in  the  spacecraft's  rotation  to 
sample  the  plasma.  Each  scan  occupied  a  small 
fraction  of  a  spin  period.  The  device  took  scans 
repeatedly,  sensing,  storing,  and  transmitting 
to  Earth  scans  for  which  it  was  optimally 
oriented.  Scans  were  typically  spaced  at  120-km 
(75-mile)  intervals  along  the  orbital  path. 

By  recording  three  scans  as  it  pointed  to  three 
different  celestial  longitudes  in  three  succes- 
sive spin  cycles,  the  instrument  measured 
vector  ion  velocity.  The  investigator  could 
command  a  special  operation  mode  to  measure 
total  ion  concentration  at  20-m  (66-ft)  intervals. 

During  Phase  II,  changing  orbital  properties 
allowed  several  series  of  observations.  Sam- 
pling at  higher  altitudes  aided  a  search  for  the 
source  of  ion  heating  in  the  nightside  iono- 
sphere. Scientists  also  investigated  how  the 



mantle  region  developed  downstream  from  the 
planet.  They  looked  for  the  source  of  nightside 
ionization  and  superthermal  electrons.  Another 
source  they  sought  was  for  ion  heating  on  the 
dayside  at  altitudes  between  150  and  170  km 
(93  and  105  miles).  Additional  information 
was  needed  about  the  nature  of  the  mantle  at 
the  subsolar  point,  how  ions  are  accelerated 
across  the  terminator,  and  characteristics  of 
the  plasma  within  flux  ropes. 

During  Phase  III,  this  experiment  provided 
information  about  the  relative  roles  of  solar 
protons  and  solar  wind  in  several  Venusian 
phenomena.  This  was  possible  because  scien- 
tists could  compare  these  measurements  with 
those  obtained  earlier  at  a  different  part 
of  the  solar  cycle. 

Gamma  Ray  Burst  Detector 
Its  designers  did  not  intend  the  gamma  ray 
burst  detector  to  obtain  information  about 
Venus.  Onboard  the  Pioneer  spacecraft  in  orbit 
about  the  planet,  the  detector  provided 
another  set  of  important  data  concerning  the 
intense  short-duration  bursts  of  high-energy 
photons  from  beyond  the  Solar  System. 
Lasting  from  one  tenth  to  a  few  tenths  of  a 
second,  these  bursts  were  first  observed  by 
scientists  in  1973.  They  occurred  randomly, 
roughly  18  bursts  each  year,  and  their  source 
was  a  mystery.  The  Orbiter  provided  a  means  to 
obtain  a  direction  for  the  bursts.  It  achieved 
this  by  correlating  observations  from  Venus 
with  simultaneous  observations  from  Earth- 
orbiting  satellites.  Several  years  of  high  quality 
observations  from  the  Pioneer  Orbiter  contrib- 
uted much  to  the  early  stages  of  these  astro- 
nomical observations. 

The  instrument  (Figure  4-11)  weighed  2.8  kg 
(6.17  Ib)  and  required  1.3  W  of  electrical 
power.  It  consisted  of  two  sodium-iodide 
photomultiplier  detector  units  to  provide  a 

near  uniform  sensitivity  over  a  wide  field  of 
view.  These  detectors  were  sensitive  to  pho- 
tons with  energies  between  0.2  and  2.0  MeV. 
To  accommodate  high  data  rates  that  occurred 
during  intense  gamma  ray  bursts,  the  experi- 
ment included  a  20-kilobit  buffer  memory 
for  storing  the  data  until  they  could  be 
telemetered  to  Earth  at  a  lower  rate. 

The  principal  investigator  for  the  gamma  ray 
experiment  was  initially  W.  D.  Evans, 
Los  Alamos  Scientific  Laboratory  (1974-1982). 
R.  W.  Klebesadel,  also  of  Los  Alamos  Scientific 
Laboratory,  replaced  him. 

Orbiter  Radio  Science  Experiments 

The  experiments  connected  with  instruments 
on  the  spacecraft  were  not  the  only  experi- 
ments. There  were  several  investigations  that 
involved  radio  signals  exchanged  between 
the  Orbiter  and  Earth.  The  team  leader  for 
these  investigations  was  G.  H.  Pettengill, 
Massachusetts  Institute  of  Technology. 

Radio  science  experiments  included  the 
following:  occultation  studies  by  A.  J.  Kliore, 
Jet  Propulsion  Laboratory,  and  T.  A.  Croft,  SRI 
International;  internal  density  distribution  of 
Venus  by  R.  J.  Phillips,  Jet  Propulsion  Labora- 
tory; celestial  mechanics  by  R.  Reasenberg, 
Massachusetts  Institute  of  Technology; 
atmospheric  and  solar-wind  turbulence  by 
R.  Woo,  Jet  Propulsion  Laboratory;  solar 
corona  by  T.  A.  Croft,  SRI  International;  and 
atmospheric  drag  by  G.  M.  Keating,  NASA 
Langley  Research  Center. 

Radio  science  experiments  used  the  space- 
craft's Doppler  tracking  system.  An  antenna 
of  the  DSN  transmitted  a  microwave  signal  at  a 
frequency  of  about  2.1  GHz.  When  the  space- 
craft received  the  signal,  it  phase-coherently 
multiplied  it  by  240/241  and  then  retrans- 


mitted  the  signal.  This  frequency  multiplica- 
tion allowed  the  spacecraft  receiver  to  detect 
the  incoming  signal  while  its  transmitter  was 
operating.  It  was  able  to  discriminate  between 
the  two  signals.  The  frequency  multiplication 
also  served  a  similar  purpose  for  the  ground 

When  the  DSN  received  the  signal,  it  mixed  it 
with  another  locally  generated  signal.  This 
process  produced  a  video  signal,  offset  by  a 
known  frequency  from  that  resulting  from  the 
Doppler  effects.  The  Doppler  shift  was  then 
reconstructed  from  this  biased  Doppler  video 
signal.  The  ground  station  counted  the  biased 
Doppler  signal  cycles.  The  differences  between 
uniformly  spaced  samples  of  the  cycle  count 
divided  by  the  count  interval  and  corrected  for 
the  effects  of  the  known  frequency  offset, 
provided  the  primary  Doppler  data.  These  data 
approximated  the  average  rate  of  change  for 
the  range  between  the  ground  station  and  the 
spacecraft,  and  thus  contained  information 
about  the  spacecraft's  acceleration. 

Most  of  the  observed  Doppler  shift  was  due  to 
the  relative  motions  of  Earth  and  Venus.  The 
mean  elliptical  trajectory  of  the  Orbiter 
accounted  for  the  greater  part  of  the  remaining 
Doppler  shift.  Scientists  attributed  a  significant 
part  of  this  Doppler  shift  to  perturbations  in 
the  spacecraft's  trajectory.  They  attributed  a 
smaller  part  to  direct  effects  of  the  propagation 
media.  Several  factors  caused  the  trajectory 
perturbations.  These  factors  included  other 
planets  and  the  Sun,  atmospheric  drag  effects, 
and  irregularities  in  Venus'  gravitational 
potential.  Analysis  of  Doppler  data  provided  a 
model  of  these  irregularities. 

The  Doppler  shift  that  the  propagation  media 
caused  had  several  components.  Each  compo- 
nent originated  from  a  different  location: 
Earth's  troposphere  and  ionosphere,  the  solar 

corona  and  plasma  that  flows  from  it,  the 
interplanetary  medium,  and,  for  some  geo- 
metries, Venus'  neutral  atmosphere  and 
ionosphere.  Some  of  the  radio  science  experi- 
ments concerned  the  characterization  of 
components  of  the  propagation  media. 

In  addition  to  transmitting  an  S-band  signal 
at  2.293  GHz,  the  spacecraft  could  transmit  an 
X-band  signal  at  8.407  GHz.  This  latter  signal 
also  was  phase  coherent  with  the  S-band 
signal.  This  X-band  signal  was  received  and 
processed  on  the  ground  in  the  same  way  as 
the  S-band  signal.  The  propagation  delay  at  a 
given  frequency  caused  by  charged  particles 
(plasma)  was  inversely  proportional  to  the 
square  of  the  frequency.  This  allowed 
investigators  to  use  the  dual-band  spacecraft 
transmissions  to  measure  the  change  of  the 
total  charged-particle  content  of  the  path  from 
the  spacecraft  to  the  ground  station. 

Figure  4-11 .  Orbiter  gamma 
ray  burst  detector  (OGBD).  The 
photograph  shows  two  photo- 
multiplier  detector  units,  one 
without  cover  to  reveal  associ- 
ated electronics.  Also,  in  the 
background  is  the  electronic 
data  processing  package. 



Internal  Density  Distribution  Experiment 
In  the  internal  density  distribution  experi- 
ment, researchers  studied  the  relationship 
between  Venus'  surface  features  and  internal 
densities.  In  their  study,  they  used  data  about 
Venus'  shape  and  the  spacecraft's  gravitational 
perturbations.  The  Orbiter's  two-way  Doppler 
tracking  data  allowed  these  researchers  to  infer 
the  planet's  gravity  field.  When  used  with 
topographic  data  obtained  from  the  radar 
experiment,  the  gravity  data  provided  a 
constraint  on  the  internal  density  distribution. 
Geophysicists  also  used  these  data.  It  allowed 
them  to  investigate  whether  there  were  any 
continuing  physical  processes  taking  place 
within  Venus  similar  to  those  moving  Earth's 
crustal  plates.  This  experiment  was  practical 
only  during  Phase  I  and  Phase  III  when 
periapsis  occurred  at  low  altitudes. 

Celestial  Mechanics  Experiment 
The  celestial  mechanics  experiment  used  the 
spacecraft's  radio  tracking  system  and  its 
onboard  radar  system.  Doppler  tracking 
produced  data  to  develop  a  high-resolution 
map  of  Venus'  gravitational  potential.  This 
map,  which  showed  the  irregularities  in 
gravity's  vertical  component  at  Venus'  surface, 
correlated  with  topography  from  the  onboard 
radar.  Researchers  could  compare  the  topogra- 
phy and  gravity  in  the  spatial  frequency 
domain.  This  comparison,  in  turn,  yielded  the 
spectral  admittance,  which  provided  a  con- 
straint on  Venus'  near  surface  structure. 
Investigators  also  used  the  Doppler  tracking 
data  to  study  the  time-variable  structure  of 
Venus'  upper  atmosphere. 

Simultaneous  radio  tracking  of  the  Orbiter 
with  extragalactic  radio  sources  allowed  precise 
determination  of  Earth  and  Venus'  orbits  with 
respect  to  these  sources.  This  experiment 
occurred  only  during  Phase  I  when  periapsis 
occurred  at  low  altitudes.  It  was  not  repeated 

during  Phase  III.  During  Phase  II,  the  increas- 
ing altitude  of  periapsis,  coupled  with  lack  of 
propulsive  maneuvers  and  atmospheric  drag, 
allowed  researchers  to  measure  the  shape  of 
the  gravitational  field  globally. 

Dual-Frequency  Radio  Occultation  Experiment 
The  dual-frequency  radio  occultation  experi- 
ment provided  information  about  Venus' 
atmosphere.  This  was  done  by  observing  how 
the  Orbiter's  S-band  and  X-band  radio  signals 
penetrated  the  planet's  ionosphere  and  neutral 
atmosphere  just  before  and  after  occultations. 
Observation  data  from  multiple  occultations 
with  Pioneer  were  very  rewarding.  There  was 
far  more  data  than  from  earlier  observations  of 
a  single  spacecraft  passing  behind  a  planet 
during  a  flyby.  Each  occultation  recorded 
Doppler  frequency  shifts  and  changes  in  signal 
strength  caused  by  refraction  and  absorption 
by  the  planet's  atmosphere. 

The  Orbiter's  repetitive  path  was  practically 
unchanging  in  orientation  to  Venus.  However, 
motions  of  Venus  and  Earth  around  the  Sun 
precessed  the  occultation  points  around  the 
limb  of  the  planet.  During  the  nominal  mis- 
sion, 80  occultations  sampled  the  atmosphere 
and  ionosphere.  Samples  were  over  all  latitudes 
from  the  North  Pole  to  about  60°  south  latitude. 
Nearly  all  observations  were,  however,  in 
Venus'  night  hemisphere.  Observations  not  in 
the  night  hemisphere  were  at  polar  latitudes. 
It  was  during  the  extended  mission  that 
investigators  acquired  data  on  the  day  side. 

During  occultations,  ground  control  aimed  the 
Orbiter's  high-gain  antenna  precisely  to  ensure 
that  radio  signals  traveled  to  Earth  after  they 
had  been  refracted  by  Venus'  atmosphere.  In 
this  way,  there  was  maximum  penetration  of 
the  atmosphere  by  the  signals.  From  this  deep 
penetration,  scientists  could  identify  and 
define  microwave  absorbing  cloud  layers. 

Analyzing  the  Doppler  frequency  variations  in 
the  radio  signals  revealed  much.  For  example, 
investigators  determined  the  structure,  the 
index  of  refraction,  temperature,  pressure, 
and  density  of  the  atmosphere  above  34  km 
(21  miles).  Radio  signal  refraction  at  Venus  is 
so  strong  that  any  level  ray  that  penetrated 
below  33  km  (20.5  miles)  curved  down  to  hit 
the  surface  and  became  useless  for  this  study. 

Atmospheric  and  Solar- Wind 
Turbulence  Experiment 
The  atmospheric  and  solar-wind  turbulence 
experiment  observed  turbulence  of  scale  sizes 
smaller  than  10  km  (6  miles)  in  Venus'  atmo- 
sphere above  34  km  (21  miles).  Experimenters 
sought  the  global  distribution  of  this  turbu- 
lence. Their  experiment  also  revealed  fluctua- 
tions in  the  ionosphere's  electron  density. 

Detailed  information  about  the  atmosphere 
was  obtained  just  before  and  after  occultation. 
At  these  times,  the  radio  signal  passed  through 
deep  regions  of  the  atmosphere  on  its  way 
from  the  Orbiter  to  Earth.  Signal  scintillations, 
akin  to  the  twinkling  of  stars  for  Earth-bound 
observers,  occurred  during  the  passage.  These 
scintillations  revealed  variations  in  the  atmo- 
sphere's density  and  the  presence  of  atmo- 
spheric layers. 

For  this  experiment,  the  ground  station  made 
a  wide-band  linear  recording  in  the  frequency 
interval  known  to  contain  the  signal.  Subse- 
quently, the  signal  was  detected  by  a  digital 
computer  simulation  of  the  phase-lock  loop  in 
a  receiver  acting  on  a  digitized  record  of  that 
wide-band  signal  plus  noise.  The  digital 
approach  was  superior  to  ordinary,  analog 
radio  signal  detection  in  many  respects.  This 
was  particularly  true  when  it  involved  critical 
scientific  applications. 

Scientists  applied  advances  in  phase  scintilla- 
tions and  spectral-broadening  measurements 
to  study  solar  wind.  They  made  these  measure- 
ments after  they  had  completed  the  nominal 
mission.  At  that  time,  Venus,  with  the  Orbiter, 
approached  superior  conjunction.  The  space- 
craft's radio  waves  then  passed  close  to  the  Sun 
on  their  way  to  Earth.  This  was  an  ideal  time 
to  investigate  solar  wind  near  the  Sun.  Because 
the  wind  is  variable,  repeated  observations 
provide  information  about  its  density,  turbu- 
lence, and  velocity.  Two  DSN  stations  simulta- 
neously recorded  fluctuations  in  the  S-band 
and  X-band  signals  as  the  signals  passed 
through  the  solar  wind. 

Scientists  compared  Pioneer  Venus  data  from 
the  inner  Solar  System  with  data  from 
Voyagers  1  and  2  and  Pioneer  Saturn  space- 
craft in  the  outer  Solar  System.  Their  compari- 
sons formed  the  basis  for  a  special  period  of 
international  collaborative  solar  corona 
observations.  This  was  the  first  scheduled 
event  of  the  Solar  Maximum  Year. 

Atmospheric  Drag  Experiment 
The  atmospheric  drag  experiment  used  drag 
measurements  made  for  the  first  time  within 
another  planet's  atmosphere.  The  aim  was  to 
model  the  upper  atmosphere's  mean  behavior. 
It  also  included  searching  for  variations  in 
atmospheric  density  that  correlated  with  solar- 
wind  activity  and  changes  in  ultraviolet 
radiation.  In  addition,  experimenters  sought 
evidence  that  the  four-day  rotation  extended 
into  the  upper  atmosphere. 

Investigators  extracted  drag  effects  from  the 
spacecraft's  estimated  orbital  parameters.  The 
navigation  team  obtained  these  parameters 
from  the  S-band  tracking  data.  By  use  of  an 
ad  hoc  model,  experimenters  determined  atmo- 
spheric density  at  each  periapsis.  This  was 
where  drag  was  greatest.  Scientists  evaluating 



the  atmospheric  density  model  relied  on  the 
periodic  variation  of  the  spacecraft's  periapsis 
altitude.  They  determined  the  drag  coefficient 
in  free  molecular  flow  from  two  observations. 
The  first  was  the  spacecraft's  orientation  rela- 
tive to  the  flightpath.  The  second  was  an 
estimate  of  the  atmosphere's  composition. 
Scientists  inferred  temperature  and  composi- 
tion variation  with  altitude  and  time  from 
several  factors.  These  were  density,  density 
scale  height,  and  knowledge  of  dominant 
atmospheric  components.  Further  analysis 
yielded  models  of  pressure  gradients  and  flow 
patterns.  Since  it  required  that  periapsis  should 
be  at  low  altitude,  this  experiment  was  useful 
during  Phase  I  and  Phase  III  only. 

Phase  III  provided  good  atmospheric  drag  mea- 
surements near  the  terminator  on  the  night- 
side.  Scientists  compared  this  information  with 
Phase  I  to  show  how  the  neutral  upper  atmo- 
sphere and  the  helium-rich  regime  above 
200  km  (124  miles)  changes.  They  also  used 
it  to  show  what  happens  to  the  lower  cryo- 
sphere's  vertical  structure  and  variability. 

Multiprobe  Bus  Experiments 

After  its  four  probes  separated  20  days  before 
reaching  Venus,  the  Multiprobe  Bus  also 
became  a  probe.  It  provided  important  infor- 
mation on  the  density  and  composition  of 
Venus'  high  atmosphere,  in  particular  for  the 
altitude  range  from  150  to  130  km  (93  to 
81  miles).  For  this  experiment,  the  Multiprobe 
Bus  carried  two  mass  spectrometer  instruments. 
Each  instrument  was  on  the  equipment  shelf 
with  its  inlet  projecting  over  the  flat  top  of  the 
spacecraft  cylinder. 

Neutral  Mass  Spectrometer 
Between  about  700  km  (435  miles)  and  130-km 
(81  miles)  altitude,  the  neutral  mass  spectrom- 
eter measured  the  components  (atoms  and 
molecules)  of  Venus'  high  atmosphere.  The  Bus 

did  not  have  protective  thermal  shields,  so 
there  was  no  way  to  prevent  or  delay  its 
destruction  by  atmospheric  heating  as  it 
plunged  at  high  speed  into  Venus'  upper 
atmosphere.  It  could  not  penetrate  much 
below  130  km  (81  miles). 

The  spectrometer  weighed  6.5  kg  (14  Ib)  and 
used  5  W  of  electrical  power.  The  principal 
investigator  for  this  experiment  was  Ulf 
von  Zahn,  University  of  Bonn,  Germany. 

From  information  gathered  by  this  instrument, 
the  investigator  derived  the  height  of  the  turbo- 
pause,  or  homopause.  Above  this  region,  the 
atmospheric  gases  do  not  mix,  but  become 
stratified  as  the  lightest  gases  congregate 
toward  the  top  of  the  atmosphere.  The  data 
also  revealed  chemical  composition  of  the 
ionospheric  region  where  density  is  greatest. 
An  additional  discovery  was  the  temperature 
of  the  exosphere,  the  atmosphere's  outer  fringe. 

The  neutral  mass  spectrometer  (Figure  4-12) 
ionized  atmospheric  components  by  bombard- 
ing them  with  electrons.  By  deflecting  them 
magnetically,  the  device  separated  the  ions 
according  to  their  masses,  up  to  46  atomic 
mass  units.  The  spectrometer  featured  a  fast 
data  sampling  and  telemetering  capability. 
This  feature  allowed  it  to  cope  with  the 
3  km/sec  (6700  mph)  speed  of  the  Bus'  vertical 
descent  (at  an  altitude  of  150  km,  or  93  miles). 
The  Bus  traveled  much  faster  when  it  first 
entered  the  atmosphere.  And,  because  it  made 
a  very  shallow  entry,  most  of  its  speed  was  in  a 
horizontal  direction. 

One  day  before  the  Bus  encountered  Venus,  a 
small  glass  vial  released  a  known  amount  of 
gas  into  the  spectrometer  to  calibrate  it.  The 
gas  provided  a  reference  to  determine  the 
instrument's  sensitivity  after  its  cruise  through 
interplanetary  space. 

The  instrument  was  a  double-focusing 
Mattauch-Herzog  electric  and  magnetic  deflec- 
tion mass  spectrometer.  Small  and  compact,  it 
provided  constant  sensitivity  at  high  pressures. 
The  design  also  permitted  use  of  a  dual  collec- 
tor system  for  a  large  dynamic  signal  range. 

The  spectrometer  had  several  major  parts.  One 
was  an  ion  source,  where  electron  bombard- 
ment ionized  atmospheric  particles.  Another 
was  an  electric  analyzer  for  mass  separation  of 
ions.  The  spectrometer  also  had  a  collector 
system,  consisting  of  multiple  elements.  These 
elements  enabled  the  system  to  collect  ions  of 
more  than  one  mass  at  the  same  time  accord- 
ing to  their  mass.  Also,  two  detectors  were 
Spiraltron  electron  multipliers.  One  detected 
ions  between  1  and  8  atomic  mass  units,  and 
the  other  detected  ions  between  12  and 
46  atomic  mass  units.  In  addition,  there  was  a 
titanium  sublimation  pump  and  an  ion  getter 
pump.  These  devices  maintained  a  pressure 
differential  of  more  than  1000  to  1  between 
the  ion  source  and  the  mass  analyzer. 

The  instrument  first  operated  in  a  peak  step- 
ping mode.  It  sampled  only  tops  of  selected 
mass  peaks  and  required  zero  levels.  However, 
below  altitudes  of  about  215  km  (134  miles), 
the  instrument  operated  for  about  25%  of  the 
time  in  a  fly-through  mode.  In  this  mode,  it 
sampled  only  high-energy  ions. 

Ion  Mass  Spectrometer 
The  Bus'  ion  mass  spectrometer  was  identical 
to  the  Orbiter's  ion  mass  spectrometer.  It 
measured  the  distribution  and  concentration 
of  positively  charged  ions  in  the  planet's  upper 
atmosphere  above  120  km  (75  miles).  The 
principal  investigator  was  H.  A.  Taylor,  NASA 
Goddard  Space  Flight  Center.  He  also  was 
principal  investigator  for  the  Orbiter's  ion 
mass  spectrometer  experiment. 

Large  Probe  Experiments 

The  Large  Probe  carried  seven  scientific  instru- 
ments. A  gas  chromatograph  and  a  mass 
spectrometer  measured  the  composition  of  the 
atmosphere  directly.  A  group  of  pressure 
sensors  measured  pressure  directly,  with  inlet 
ports  penetrating  the  probe's  shell.  The  other 
five  instruments  observed  through  the  probe's 
windows  and  sensed  the  probe's  motion.  They 
also  measured  temperature  through  externally 
mounted  sensors. 

An  infrared  radiometer  required  a  diamond 
window  because  diamond  was  the  only 
material  transparent  to  the  wavelengths  of 
interest.  It  also  was  the  only  material  capable 
of  withstanding  the  high  temperatures  and 
pressures  within  Venus'  lower  atmosphere.  The 
window  was  about  1.9  cm  (0.75  in.)  in  diam- 
eter and  0.32  cm  (0.125  in.)  thick,  or  about  the 
size  of  a  quarter.  It  weighed  13.5  carats.  Dia- 
mond cutters  in  the  Netherlands  shaped  it 
from  a  205-carat  industrial-grade,  rough 
diamond  from  South  Africa. 

A  nephelometer  used  two  sapphire  windows.  A 
cloud  particle  instrument  also  used  a  sapphire 
window,  directing  a  laser  beam  through  it.  The 
beam  traveled  to  an  outside  reflecting  prism 
and  then  back  to  its  sensor.  A  solar  flux 
radiometer  used  five  sapphire  windows. 

Neutral  Mass  Spectrometer 
The  neutral  mass  spectrometer  (Figure  4-13) 
measured  the  composition  of  the  lower  62  km 
(38  miles)  of  Venus'  atmosphere.  This  region 
was  mostly  below  the  cloud  layers.  Informa- 
tion on  the  relative  abundance  of  gases  in  this 
region  was  important.  With  it,  scientists  could 
better  understand  the  planet's  evolution, 
structure,  and  heat  balance. 

The  spectrometer,  which  weighed  10.9  kg 
(24  Ib)  and  required  14  W  of  electrical  power, 


Ion  box 
Ion  repeller 

Ambient  gas  r\    • 
particles  I/  ! 

Electron  beam 

Total  ion 
current  monitor- 
Ion  focusing 
plates  /+,  I- 

Object  slit 

Ion  beam 

Low  mass  ion 
(1-8  AMU) 

Magnetic  analyzer 

Electric  analyzer 
Ion  retarding  slit 

Electron  focusing 

Low  mass 

High  mass 
ion  collector 




High  mass 

Cap  eject 


IP  on  cmd 

Data  line  1 
Data  line  2 

TP  on  cmd 

16-bit  cmd 

S/C  timing 

Cal  gas 


106      Figure  4-12.  Multiprobe  Bus 
neutral  mass  spectrometer 
(BNMS).  (Top  left)  Mass 
spectrometer.  (Bottom  left) 
Electronics  package.  (Top 
right)  Diagram  to  illustrate 
ionized  gas  path  through 
the  instrument  to  detectors. 
(Bottom  right)  Schematic  of 
the  instrument  and  its 

consisted  of  two  units.  Both  units  were  on  a 
single  baseplate  on  the  probe's  lower  shelf.  A 
mass  analyzer,  ion  source,  pumping  system, 
isotope  ratio  measuring  cell,  and  valves  were 
in  one  unit.  Electronics  were  in  the  other.  The 
principal  investigator  was  J.  H.  Hoffman, 
University  of  Texas,  Dallas. 

The  instrument  had  wide  dynamic  and  mass 
ranges  to  survey  atmospheric  gases  and  deter- 

mine cloud  composition.  Its  design  made  sure 
that  the  sampling  process  did  not  alter  chemi- 
cally active  species.  To  prevent  such  alteration, 
it  collected  samples  through  a  chemically 
passive  inlet  leak. 

The  inlet  consisted  of  a  pair  of  microleaks, 
each  formed  by  compressing  the  tip  of  a 
tantalum  tube  into  a  slit.  The  tubes  projected 
through  the  probe  wall  to  beyond  the 

EM  high-voltage 
power  supply 

Microprocessor  logic  unit 

CML  heater 
power  supply 

Monitor  and  house- 
keeping subassembly 

power  supply 

.___: ., ,____*_ 

Probe  power 

Multiplexer  analog  signal 
TM  word  30 


VCV  —  Variable  conductance  valve 
EM  —  Electron  multiplier 
Ve  —  Electron  multiplier  high  voltage 
+Vs  —  Sweep  high  voltage 
G  —  Getter 
IP  —  Ion  pump 

Data  output 


6,  8,  12, 



timing  signals 

IS  —  Ion  source 
S  —  Molecular  sieve 
o  — Valve 
^  —  Filament 

=  —  Electron  beam 

Figure  4-13.  Large  Probe  neutral 
mass  spectrometer  (LNMS). 
(Left)  Schematic  diagram  of  the 
instrument  showing  various 
functions  in  the  operation  of  the 
spectrometer  and  its  associated 
electronics.  (Right)  Photograph 
of  assembled  unit. 



boundary  layer.  When  the  atmospheric  pres- 
sure reached  1.5  bars,  the  tube  with  the  larger 
conductance  closed  off.  This  prevented  too 
large  a  sample  deeper  within  the  atmosphere 
when  pressure  increased  rapidly.  Atmospheric 
gases  and  vapors  were  pumped  into  an  ion 
source  through  a  variable  conductance  valve. 
During  descent,  the  valve  gradually  opened  to 
keep  a  constant  pressure  at  the  ion  source.  A 
magnetic  sector  field  mass  spectrometer 
analyzed  the  gas  sample.  Its  range  was  1  to 
208  atomic  mass  units.  The  spectrometer 
detected  minor  constituents  in  1-ppm  concen- 
tration over  the  entire  descent.  To  identify 
unknown  substances  and  separate  parent  peaks 
from  fragmentary  ions,  ionizing  electron 
energy  was  stepped  through  three  levels. 

Each  mass  spectrum  took  64  seconds  to 
sample.  A  microprocessor  controlled  the  mass 
scan  mode,  sequencing  of  ion  source  energy, 
and  data  accumulation  and  formatting.  The 
instrument  converted  accumulated  counts  for 
each  spectral  peak  into  10-bit,  base-2,  floating- 
point numbers.  With  a  rate  of  only  40  bits/sec, 
the  spacecraft  successfully  transmitted  to  Earth 
data  from  about  50  spectra  obtained  during 
the  descent. 

The  instrument  used  an  isotope  ratio  measur- 
ing cell  to  collect  a  sample  shortly  after  the 
parachute's  deployment.  In  this  cell,  the 
sample  was  purged  of  carbon  dioxide  and 
other  active  gases.  After  purging,  an  enriched 
sample  of  inert  gases  was  left.  Then  the  device 
pumped  out  the  ion-source  cavity  and  ana- 
lyzed the  sample  to  determine  the  isotope 
ratios  of  such  inert  gases  as  xenon,  argon,  and 
neon.  All  these  gases  are  important  for  under- 
standing how  Venus'  atmosphere  evolved. 

Gas  Chromatograph 

The  gas  chromatograph  experiment  also  mea- 
sured the  gaseous  composition  of  Venus'  lower 

atmosphere.  It  was  a  modified  version  of  the 
gas  exchange  experiment  the  Viking  lander 
carried  to  Mars  in  1976.  It  measured  gases 
likely  to  be  on  Venus,  with  the  aim  of  answer- 
ing questions  about  Venus'  evolution,  struc- 
ture, and  thermal  balance.  The  principal 
investigator  was  V.  Oyama,  NASA  Ames 
Research  Center. 

The  instrument  (Figure  4-14)  weighed  6.3  kg 
(13.9  Ib)  and  required  42  W  of  electrical  power. 
It  sampled  the  lower  atmosphere  three  times 
during  the  Large  Probe's  descent.  During  each 
sampling  process,  atmosphere  flowed  through 
a  tube  into  a  helium  gas  stream.  This  stream 
swept  the  sample  into  two  chromatograph 
column  assemblies.  There  the  device  identified 
atmospheric  components  by  the  time  each 
took  to  flow  through  the  columns. 

A  long  column  assembly  consisted  of  a  matched 
pair  of  1585-cm  (624-in.)  packed  columns  bifi- 
larly  wound.  Each  column  contained  polystyrene 
(Porapak  N)  and  operated  at  18°C  (64°F).  A  pro- 
portional heater  surrounded  by  a  shell  of  phase 
change  material  (n-hexadecane)  controlled  the 
temperature.  The  long  columns  were  for  gases 
with  masses  between  those  of  neon  and 
carbon  dioxide. 

A  short  column  assembly  also  was  part  of  the 
instrument.  It  consisted  of  similarly  wound 
244-cm  (96-in.)  columns.  These  columns  con- 
tained a  mixture  of  polymer  spheres  (80% 
polydivinyl  benzene,  20%  ethylvinyl-benzene). 
The  materials  remained  at  an  operating  tem- 
perature of  62°C  (144°F).  These  short  columns 
were  for  gases  in  the  mass  range  from  carbon 
dioxide  to  sulfur  dioxide. 

As  the  gases  sequentially  emerged  from  the 
columns,  they  passed  to  a  thermal  conductiv- 
ity detector  that  generated  data.  These  data 
remained  in  a  buffer  memory  awaiting 


Helium  carrier 
gas  supply 


Long  column 




Test  ports 


valves  (5) 


Mag-thorium  housing 
Circuit  boards  (6) 

Interslice  shield 






MSS/ESS  interface  connector 
Short  columns 


Cas  sample 



Inlet  line 

telemetry.  As  a  calibration  check,  two  samples 
of  Freon  (a  gas  not  likely  to  be  in  Venus'  atmo- 
sphere) were  added  to  each  third  sample. 

Solar  Flux  Radiometer 

The  solar  flux  experiment  measured  the  height 
of  the  region  in  Venus'  atmosphere  where  solar 
energy  is  deposited  to  heat  the  atmosphere. 
The  principal  investigator  was  M.  Tomasko, 
University  of  Arizona.  The  radiometer 
(Figure  4-15)  weighed  1.6  kg  (3.5  Ib)  and 
required  4  W  of  electrical  power.  It  revealed  how 
much  sunlight  clouds  absorbed  and  how  much 
reached  the  surface.  This  information  was 
important  for  understanding  Venus'  heating 
mechanism.  Does  heat  result  from  a  greenhouse 
effect  where  the  planet  absorbs  solar  energy 
efficiently  but  reradiates  it  inefficiently? 

Figure  4-14.  Large  Probe  gas 
chromatograph  (LCC).  (Top) 
Cutaway  drawing  of  instru- 
ment showing  its  major  pans. 
(Bottom)  Photograph  of 
assembled  instrument. 


Figure  4-15.  Large  Probe  solar 
flux  radiometer  (LSFR).  (Top) 
Detailed  diagram  of  the  instru- 
ment's detector  head  shows  the 
location  of  quartz  lenses,  light 
pipes,  and  filters.  (Bottom) 
Photograph  of  assembled  unit: 
detector  head  on  left  and 
electronics  package  on  right. 




Light  pipe 

Lens  block 

LiNOj  •  3H2O 
phase  change 

The  instrument  continually  measured  the 
difference  in  intensity  of  sunlight  directly 
above  and  below  the  probe's  horizon.  Five 
quartz  lenses,  3  mm  (0.125  in.)  in  diameter, 
inside  five  flat  sapphire  windows  collected  the 
light  and  transmitted  it  along  quartz  rods  to  a 
detector  array  of  12  separate  photovoltaic 

detectors.  The  intensity  of  sunlight  was  detected 
over  the  spectral  range  of  0.4  to  1.8  microns. 
This  is  where  83%  of  solar  energy  is  concen- 
trated. Two  broad  and  flat  spectral  channels 
were  included  at  each  azimuth  and  zenith 
sample.  One  filtered  a  channel  from  0.4  to 
1.0  microns,  the  other  a  channel  from  1.0  to 

1.8  microns.  The  instrument  also  used  a 
narrow  filter  from  0.6  to  0.65  microns  at  one 
of  the  upward-looking  zenith  samples  and  one 
of  the  downward-looking  samples.  This  chan- 
nel provided  information  about  the  single 
scattering  albedo  and  the  clouds'  optical  depth 
along  the  descent  path. 

A  mass  of  phase-change  lithium  salt,  which 
absorbed  heat  as  it  melted,  cooled  the  detector 
array.  The  detector  head  consisted  of  lenses, 
quartz  rods,  filters,  detectors,  and  their  sup- 
porting structure.  It  had  12  electronic  chan- 
nels, and  the  electronics  package  contained 
12  logarithmic  amplifiers  for  these  channels. 

Mission  scientists  were  concerned  that  either 
the  probe  or  the  parachute  might  affect  the 
measurements.  To  avoid  this,  engineers 
restricted  the  instrument's  field  of  view  to  a 
narrow  5°  over  a  carefully  selected  set  of 
azimuth  and  zenith  angles. 

The  instrument  operated  in  two  modes.  At  the 
start,  it  detected  the  intensity  peak  at  the  solar 
azimuth.  It  used  the  time  of  successive  peaks 
to  control  a  mode-1  azimuth  sampling  accord- 
ing to  preset  values.  If  a  period  of  16  seconds 
passed  without  detecting  a  peak,  the  instru- 
ment then  automatically  switched  to  a  second 
mode.  In  mode  2,  it  collected  samples  at  each 
zenith  angle  as  frequently  as  the  telemetry  rate 
allowed,  which  was  every  8  seconds.  This 
provided  a  vertical  resolution  of  300  m  (984  ft), 
or  2.67  times  better  than  mode-1  resolutions. 
When  the  probe  penetrated  to  an  altitude  of 
54  km  (34  miles),  the  instrument  locked  into 
mode  2  for  the  rest  of  the  descent. 

Infrared  Radiometer 

The  infrared  radiometer  (Figure  4-16)  mea- 
sured vertical  distribution  of  infrared  radiation 
in  the  atmosphere.  It  took  measurements  from 
the  time  the  Large  Probe's  parachute  deployed 

until  the  probe  reached  the  planet's  surface.  It 
also  detected  cloud  layers  and  water  vapor, 
both  important  traps  for  solar  heat.  The  instru- 
ment weighed  2.6  kg  (5.8  Ib)  and  required 
5.5  W  of  electrical  power.  The  principal 
investigator  for  this  experiment  was  R.  Boese, 
NASA  Ames  Research  Center. 

The  radiometer  consisted  of  two  sections:  an 
optical  head  and  an  electronics  box.  On  the 
aft  side  of  the  probe's  forward  shelf,  it  gathered 
information  through  a  diamond  window.  The 
window  was  heated  to  prevent  contamination 
during  descent  through  the  clouds.  It  provided 
an  unobstructed  conical  field  of  view  of  25°  cen- 
tered at  45°  upward  and  downward  from 
the  horizontal. 

Designers  chose  six  pyroelectric  infrared  detec- 
tors. Because  they  required  no  special  cooling 
equipment,  they  were  well  suited  to  Venus'  high 
temperatures.  Each  detector  viewed  the  atmo- 
sphere through  rotating  light  pipes  (to  minimize 
stray  light).  They  also  used  a  different  infrared 
filter  between  3  and  50  microns.  These  detectors 
possessed  uniform  sensitivity  throughout  the 
infrared  range.  Although  the  detectors  needed 
no  protection  from  heating,  preamplifiers,  which 
were  closely  connected  to  them,  did  need  protec- 
tion. So  phase-change  material  was  put  around 
the  detector  package  to  control  temperature. 

Filters  for  the  6  channels  covered  these 
ranges:  3  to  50  microns,  6  to  7  microns,  7  to 
8  microns,  8  to  9  microns,  14.5  to  15.5  microns, 
and  4  to  5  microns.  The  first  channel  allowed 
measurement  of  the  entire  thermal  flux.  The 
next  two  channels  searched  for  water  vapor. 
The  fourth  channel  provided  information  on 
cloud  opacity.  The  fifth  channel,  centered  in  a 
strong  band  of  carbon  dioxide,  revealed  any 
obscurities  of  the  outer  window.  The  sixth 
band  determined  window  temperature. 


Extension  pipe 

—  Rotating  pipe 

Stationary  pipe 

—  D  iff  user 

Figure  4-16.  Large  Probe  infrared 
radiometer  (LIR).  (Above)  Photo- 
graph of  assembled  unit.  (Top 
right)  Simplified  diagram  to 
show  instrument's  major  com- 
ponents. (Bottom  right)  Cutaway 
of  detector  and  filter  package 
identifying  various  parts. 


Phase  change 
heat  sink 

Alignment  flat 

Main  electronics 




Motor  and  heater 
drive  logic 

black  body  - 


Heat  sink 
fill  plug 

Position  resolver 

Detector  preamplifiers 
Detector  array 

Two  black  bodies  within  the  instrument  pro- 
vided a  calibration  system.  These  remained  at 
temperatures  sufficiently  different  to  generate 
a  signal-to-noise  ratio  of  at  least  100:1.  This 
happened  in  all  the  detector-filter  channels. 
The  instrument  was  commanded  into  this 

calibrate  mode  approximately  6%  of  the  time 
during  descent. 

An  electronics  box  conditioned  power  from 
the  spacecraft's  electrical  system.  This  enabled 
it  to  provide  closely  regulated  voltages  that 


items  within  the  instrument  needed.  It  also 
conditioned  the  output  signals  from  the  detec- 
tors and  prepared  data  for  telemetry  to  Earth. 

Vertical  resolution  within  Venus'  atmosphere 
varied  from  about  260  m  (853  ft)  at  the  top 
of  the  atmosphere  to  about  90  m  (295  ft)  near 
the  surface.  The  telemetry  bit  rate  assigned  to 
the  experiment  governed  the  resolution, 
which  allowed  integration  of  data  over  a 
six-second  period. 

Cloud  Particle  Size  Spectrometer 
The  cloud  particle  size  spectrometer  (Figure  4-17) 
measured  sizes,  shapes,  and  densities  of  par- 
ticles within  clouds  and  in  the  lower  atmo- 
sphere. R.  Knollenberg,  Particle  Measuring 
Systems,  Inc.,  directed  the  investigation.  By 
measuring  particle  size  and  mass,  the  investga- 
tion  provided  a  vertical  profile  of  particulate 
concentration  for  34  different  size  classes. 
These  categories  ranged  from  1  to  50  microns. 
Such  measurements  provided  clues  to  basic 
cloud  formation  processes  and  interactions 
between  clouds  and  sunlight.  The  spectrom- 
eter also  determined  if  there  were  ice  crystals. 
It  did  this  by  determining  if  particles  had  the 
typical  ratio  of  particle  thickness  to  size  for  ice. 
In  this  way,  the  instrument  could  tell  them 
apart  from  other  crystal-like  particles. 

With  this  instrument,  investigators  could 
resolve  the  heights  of  clouds  to  within  400  m 
(1312  ft).  Its  prime  measuring  technique  was 
optical  array  spectrometry.  This  technique 
covered  particle  sizes  in  sequential  ranges  of 
5  to  50  microns,  20  to  200  microns,  and  50  to 
500  microns.  It  used  multiplexed  photodiode 
arrays  to  achieve  this.  Each  size  range  included 
10  size  classes  of  equal  size  width.  Also,  a  scat- 
tering subrange  used  one  of  the  light  paths  to 
measure  particle  sizes  from  0.5  to  5  microns. 

The  instrument,  weighing  4.4  kg  (9.6  Ib)  and 
requiring  20  W  of  electrical  power,  directed  a 
laser  beam  onto  an  external  prism.  The  prism 
was  supported  15  cm  (6  in.)  from  the  outer 
wall  of  the  probe's  pressure  vessel.  A  metal 
flexible  bellows  mechanically  decoupled  it 
from  the  wall.  The  prism  directed  the  laser 
beam  back  into  the  pressure  vessel  to  a  back- 
scatter  detector.  There,  a  system  of  lenses  and 
beam  splitters  generated  three  independent 
optical  paths.  When  a  particle  entered  the 
instrument's  field  of  view,  its  shadow  was  cast 
onto  a  photodiode  array  detector.  The  instru- 
ment measured  and  recorded  the  shadow's 
size.  Another  way  of  measuring  particle  size 
used  light  scattered  by  single  particles.  This 
process  resolved  5-micron  particles.  A  third 
measurement  of  particle  transit  time  gave  the 
average  thickness  of  the  particle.  (Particle 
transit  time  is  the  time  a  particle  needs  to  pass 
through  the  beam.) 

Experiments  Common  to  Large  and 
Small  Probes 

There  were  two  experiments  common  to  the 
three  Small  Probes  and  the  Large  Probe.  These 
were  the  atmospheric  structure  experiment 
and  the  nephelometer  experiment.  Each  of  the 
four  probes  carried  identical  instruments  for 
these  experiments. 

Atmospheric  Structure  Experiment 
The  atmospheric  structure  experiment  was 
aimed  at  finding  the  structure  of  Venus' 
atmosphere  from  200  km  (124  miles)  down  to 
the  surface.  It  involved  four  well-separated 
entry  sites.  Temperature,  pressure,  and  accelera- 
tion sensors  on  all  four  probes  yielded  data. 
These  data  included  location  and  intensity  of 
atmospheric  turbulence  and  temperature 
variation  with  pressure  and  altitude.  The 
atmosphere's  average  molecular  weight  and 
the  radial  distance  from  the  planet's  center 


Particle  shadow 

Figure  4-17.  Large  Probe  cloud 
particle  size  spectrometer  (LCPS). 
(Above)  Photograph  of  the 
assembled  spectrometer.  (Top 
right)  Diagram  of  optical  path. 
(Bottom  right)  Block  schematic  of 


Folding  mirrors 

Secondary  lenses 

light  deflection 


Rhomboid  prism 

Beamsplitter  prism 

Beamshaping  lenses 






Roofed  porro  prism 

ED  =  end  diode 
D10  =  diode  No.  10 
D1  =  diode  No.  1 
Disc  =  discriminator 
PA  =  preamp 
Int  =  intensity 

Voltage  _4 

monitor  — 

Temp  4 


I  Programmer^—  s/c  c|ock 



also  were  among  the  data.  A.  Seiff,  NASA 
Ames  Research  Center,  was  the  principal 

The  Large  Probe's  instruments  for  this  experi- 
ment weighed  2.3  kg  (5.1  Ib)  and  required 
4.9  W  of  electrical  power.  The  instruments  on 
each  Small  Probe  weighed  1.2  kg  (2.7  Ib)  and 
required  3.5  W  of  electrical  power  (Figure  4-18). 

The  temperature  sensors  were  dual  resistance 
thermometers.  Each  had  one  free  wire  element 
protruding  into  the  atmosphere  for  maximum 
sensitivity.  Another  wire  element  was  bonded 
to  the  support  frame  for  maximum  survivabil- 
ity.  The  sensors  could  record  temperatures 
from  -100°C  (-148T)  to  525°C  (977°F).  A 
current  source  of  10  mA,  constant  to  within 
20  ppm,  stimulated  the  sensor.  The  potential 
drop  across  the  sensor  measured  temperature. 

The  pressure  sensors  were  multiple-range, 
miniature,  silicon-diaphragm  sensors.  They 
had  to  operate  over  a  wide  dynamic  range 
from  30  mbars  to  100  bars.  To  meet  this  require- 
ment, the  device  used  12  sensors,  each  cover- 
ing a  small  pressure  range.  These  sensors  were 
sampled  in  a  way  that  preserved  data  even  if 
one  did  not  work  properly.  Each  sensor  had  a 
strain  element,  diffusion-bonded  onto  the  pres- 
sure side  of  the  diaphragm.  Engineers  arranged 
the  four  resistors  as  a  Wheatstone  bridge.  Two 
resistors  could  deform,  two  could  not. 

Engineers  developed  acceleration  sensors  (four 
on  the  Large  Probe,  one  on  each  Small  Probe) 
from  highly  accurate  guidance  accelerometers. 
They  used  a  pendulous  mass  maintained  in  a 
null  position.  Interaction  of  a  current  in  a  coil 
inside  the  mass  with  a  permanent  magnetic 
field  made  this  possible. 

The  amount  of  current  needed  to  keep  the 
mass  in  the  null  position  was  a  measure  of  the 

acceleration.  By  changing  load  resistors  and 
amplifier  gain,  the  sensors  could  switch  over  a 
range  from  0.4  microgravity  to  600  gravities. 
The  spacecraft  used  four  ranges  during  entry 
and  two  during  descent. 

An  electronics  package  distributed  power  to 
the  sensors,  sampled  their  output,  and  changed 
their  ranges.  It  also  stored  their  data,  ready  for 
telemetry.  There  were  separate  data  formats  for 
the  high-speed  entry  phase,  transition  to  the 
descent  phase,  the  descent  phase  itself,  and 
use  on  the  surface  if  the  probe  survived. 


The  nephelometer  (Figure  4-19)  searched  for 
cloud  particles.  The  objective  was  to  find  out  if 
cloud  layers  vary  from  location  to  location,  or 
if  they  were  uniformly  distributed  around  the 
planet.  By  providing  all  four  probes  with  a 
nephelometer,  investigators  were  able  to  resolve 
such  questions.  Each  instrument  weighed  1.1  kg 
(2.4  Ib)  and  required  2.4  W  of  electrical  power. 
The  experiment's  principal  investigators  were 
B.  Ragent,  NASA  Ames  Research  Center,  and 
J.  Blamont,  University  of  Paris,  France. 

To  investigate  cloud  particles,  the  nephelom- 
eter used  a  solid-state,  light  emitting  diode 
(LED)  operating  at  9000  angstroms.  The  LED 
illuminated  the  surrounding  Venusian  atmo- 
sphere near  the  probe  (but  beyond  the  aerody- 
namically  disturbed  region).  The  device 
measured  the  intensity  of  light  backscattered  by 
atmospheric  particles.  On  those  probes  enter- 
ing the  sunlit  hemisphere,  the  instrument  also 
measured  background  solar  light  penetrating 
the  atmosphere.  It  made  measurements  at 
two  wavelengths:  3550  angstroms  and 
5200  angstroms.  The  LED  illuminated  the 
atmosphere  through  a  window  in  the  probe's 
pressure  vessel.  Through  a  second  window, 
receivers  measured  intensity  of  backscattered 
light  and  background  solar  light.  A  plastic 


25  urn  wire  sensor 
bonded  to  front  of 
Pt  tubing 

Free  wire  sensor, 
0.1  mm  Pt  wire 

Frame,  Pt 
RH  tubing 


stainless  steel 
support  post 




Gas  flow 


Gas  flow 

Small  probes 

Figure  4-18.  Probe  atmospheric 
structure  experiment  (LAS/SAS), 
on  all  probes.  (Left)  Photograph 
of  instrument.  The  letters  identify: 
A,  multirange  atmospheric 
pressure  sensor;  B,  single-axis 
accelerometer;  C,  electronics 
package;  D,  one  multirange 
sensor;  E,  electronics  package. 
(Right)  Diagram  of  one  tempera- 
ture sensor  and  its  location  on 
the  probes. 


Fresnel  lens  focused  the  beams.  Investigators 
fixed  calibration  targets  to  the  Small  Probes' 
window  covers  and  to  the  Large  Probe's 

The  instrument  consisted  of  an  optical  subsys- 
tem and  an  electronics  subsystem.  The  former 
consisted  of  two  major  optical  trains  of  elements: 
a  transmitter,  a  receiver,  and  a  lens  barrel  for 
each.  A  fiber  optics  light  pipe,  shielded  from 
direct  reflections,  conducted  some  of  the  light 
reflected  from  the  front  surface  of  the  window 
through  which  transmitted  light  passed  from 
the  probe.  The  system  used  this  light  pipe  to 
monitor  the  state  of  the  window  and  the  con- 
dition of  the  light-emitting  diode.  Three 
solid-state  photodiodes  detected  backscattered 
light,  ultraviolet  background,  and  visible  back- 
ground. The  lens  barrels  for  each  channel  gave 
some  thermal  insulation  and  also  collimated 
the  light.  Borosilicate  glass  elements  provided 
further  thermal  insulation. 

The  electronic  subsystem  converted  electrical 
power  for  the  instrument.  It  provided  timing 

and  logic  control  and  conditioned  the  LED 
pulse  power.  It  also  compressed  data  and 
prepared  it  for  telemetry.  Digital  data  tele- 
metered to  Earth  included  measurements  of 
backscattered  light  and  calibration  and  moni- 
toring data.  These  data  included  temperature, 
channel  noise,  and  the  window's  condition. 
Investigators  used  the  experiment  to  construct 
a  vertical  profile  of  particle  distribution  in  the 
lower  atmosphere.  The  two  Small  Probes 
descending  on  the  planets'  sunlit  side  also 
measured  vertical  distribution  of  scattered 
solar  light  in  the  ultraviolet  and  visible  regions 
of  the  spectrum. 

Small  Probe  Experiment 

One  experiment  was  exclusive  to  the  Small 
Probes — the  net  flux  radiometer  experiment. 
It  mapped  planetary  positions  of  sources  and 
absorbers  of  radiative  energy  and  their  vertical 
distribution.  This  experiment  enhanced  our 
understanding  of  what  powers  Venus'  atmo- 
spheric circulation.  The  principal  investigator 
for  this  experiment  was  V.  E.  Suomi,  Univer- 
sity of  Wisconsin. 

The  instrument  (Figure  4-20)  weighed  1.1  kg 
(2.4  Ib)  and  required  3.8  W  of  electrical  power. 
It  consisted  of  a  sensor  assembly  outside  each 
Small  Probe's  pressure  vessel.  This  assembly, 
inside  a  protective  enclosure,  was  deployed  only 
after  the  probe  experienced  its  maximum  decel- 
eration during  atmospheric  entry.  The  sensor 
was  a  net  flux  detector  on  an  extension  shaft 
that  could  rotate  periodically  through  180°. 
This  rotation  canceled  offsets  of  the  instru- 
ment and  reduced  asymmetric  heating  effects. 
The  detector  also  included  a  temperature  sen- 
sor and  a  heater.  The  latter  reduced  condensa- 
tion on  the  detector's  diamond  windows.  The 
windows — two  per  detector — were  cut  from  the 
same  stone  as  the  infrared  radiometer  window. 

The  flux  plate  was  parallel  to  Venus'  surface.  A 
difference  between  upward  and  downward 
radiant  energy  falling  on  the  two  sides  of  the 
flux  plate  produced  a  temperature  gradient 
through  it.  This  induced  an  electric  current,  a 
measure  of  the  flux  difference.  The  plate  was 
flipped  through  180°  every  second. 

An  electronics  module  processed  two  flux 
parameters.  These  parameters  were  the  integral, 
time-averaged  flux  and  the  maximum  and 
minimum  values  of  a  periodic  input.  Internal 
timing  controlled  the  system,  which  operated 
over  four  dynamic  ranges.  In  addition  to  sci- 
ence measurements,  the  instrument  performed 
other  duties.  For  example,  it  transmitted  detec- 
tor housing  temperature,  amplifier  tempera- 
ture, and  status  of  the  detector  and  its  heater. 

Multiprobe  Radio  Science 
As  with  the  Orbiter,  scientists  used  radio 
signals  from  the  Multiprobe  mission  (probes 
and  Bus)  for  several  experiments  that  did  not 
require  instruments  onboard  the  spacecraft. 
These  were  a  differential,  long-baseline 
interferometry  experiment,  an  atmospheric 

Figure  4-19.  Probe  nephelometer 
(LN/SN)  on  all  probes.  (Top) 
Photograph  of  nephelometer 
showing  its  compact  form. 
(Middle)  Instrument's  various 
components.  (Bottom)  Optical 
path  through  the  instrument. 



Laser  high-voltage    N.     \V        I 
converter  ^^  ^\\       / 

>  \f-i 




Main  electronics 

and  low-voltage 



£  —  Large  Probe 

^J  '\^  equator 

^-  Pressure  vessel 

Folding  mirrors 


Secondary  lenses 

Laser  /—  Rhomboid  prism 

Beamsplitter  prism 

—  Beamshaping 



External  prism 


Figure  4-20.  Small  Probe  net 
flux  radiometer  (SNFR)  on  each 
small  probe.  (Top)  Photograph 
of  sensor.  (Middle)  Diagram 
showing  details  of  sensor 
assembly  and  its  components. 
(Below)  Photograph  of  associ- 
ated electronics  package. 


I1NIVI  KM)  Y      111       Wl'.l.tlN 
Nt   I      'i  Hk     UMliUMi    II 

I  '  .Nl  U  1 
I. UN  I  MAI.  I      NA'i     .      I'M 


r       "^ 

>—  Extensk 


)  Detector 


Heater  and 



Window     windings 





propagation  experiment,  and  an  atmospheric 
turbulence  experiment.  The  principal  investi- 
gators for  these  experiments  were,  respectively, 
C.  C.  Counselman,  Massachusetts  Institute  of 
Technology,  T.  A.  Croft,  SRI  International,  and 
R.  Woo,  Jet  Propulsion  Laboratory. 

Differential  Long-Baseline  Interferometry 
The  differential  long-baseline  interferometry 
experiment  measured  wind  velocity  and  direc- 
tion in  Venus'  atmosphere.  This  measurement 
occurred  as  the  four  probes  descended  through 
the  atmosphere.  Experimenters  compared  the 
probes'  descent  paths  with  simultaneous  mea- 
surements of  atmospheric  temperature  and 
pressure  from  probe  sensors.  This  information 
was  to  help  develop  an  improved  model  for 
atmospheric  circulation. 

While  the  four  probes  descended  to  the  sur- 
face, the  Multiprobe  Bus  remained  above  the 
atmosphere.  It  followed  a  ballistic  trajectory 
that  scientists  could  determine  accurately 
relative  to  the  planet.  Probe  velocities  were 
measured  differentially  with  respect  to  the  Bus, 
while  velocities  relative  to  the  planet  were 
determined  by  reference  to  the  known  Bus 
trajectory.  Probe  trajectory  deviations  from  the 
mathematical  model  in  a  still  atmosphere  were 
attributed  to  winds. 

Two  DSN  stations,  Goldstone  and  Canberra, 
and  two  Spaceflight  Tracking  and  Data 
Network  stations,  Santiago  and  Guam,  tracked 
all  spacecraft  at  the  same  time.  Experimenters 
inferred  the  component  of  the  velocity  vector 
along  the  Earth-Venus  line  of  sight  from  the 
received  signals'  Doppler  frequency  shifts.  To 
find  the  other  two  components  of  each  probe's 
velocity  vector,  they  used  differential 
long-baseline  interferometry. 

Atmospheric  Propagation  Experiment 
The  atmospheric  propagation  experiment 
attempted  to  obtain  information  about  the 
surface  and  the  atmosphere.  It  did  this  by 
studying  the  effects  of  the  atmosphere  on  the 
probes'  radio  signals.  As  the  probes  descended, 
some  of  the  transmitted  power  from  the 
relatively  broad  antenna  beam  reflected  from 
the  planet's  surface.  Doppler  effects  shifted 
this  signal  away  from  the  probe  signal  by  up  to 
200  Hz.  Since  they  provided  a  second  compo- 
nent of  the  Doppler  shift  from  a  different  angle, 
these  reflections  provided  information  about 
atmospheric  winds.  Data  also  came  from  atmo- 
spheric refraction  and  attenuation  due  to  clouds. 

Atmospheric  Turbulence  Experiment 
The  atmospheric  turbulence  experiment,  which 
R.  Woo,  Jet  Propulsion  Laboratory,  directed, 
studied  turbulence  in  Venus'  atmosphere.  It 
achieved  this  by  observing  scintillations  of  the 
probes'  radio  signals  as  each  probe  penetrated 
deep  into  the  atmosphere.  These  data  com- 
plemented the  radio  scintillation  measure- 
ments made  above  35  km  (22  miles)  during 
Orbiter  occultations. 

Interdisciplinary  Scientists 

For  the  Pioneer  Venus  program,  mission  offi- 
cials selected  several  interdisciplinary  scientists 
for  both  the  Multiprobe  and  Orbiter  missions. 
These  scientists  helped  analyze  Venus'  envi- 
ronment and  generate  a  broader  picture  of  the 
results  from  individual  experiments. 

Pioneer  Venus  was  the  first  NASA  program  to 
formally  select  interdisciplinary  scientists  for 
participation  from  a  program's  beginning.  The 
objective  was  to  include  senior  scientists  with 
a  broad  perspective  cutting  across  disciplines 
represented  by  individual  experiments. 
Mission  personnel  viewed  the  science  payload 



of  the  Pioneer  Venus  Orbiter  as  an  integrated 
set  of  instruments.  This  set  was  to  address 
more  global  scientific  questions  than  any 
single  experiment  could  handle.  The  inter- 
disciplinary scientists  played  major  roles  in 
producing  the  mission's  scientific  results.  They 
also  assumed  key  management  and  advisory 
roles  in  the  project  and  program  offices,  and  in 
the  Science  Steering  Group. 

The  tasks  of  these  scientists  included  serving  as 
members  of  a  continuing  Science  Steering 
Group  throughout  the  nominal  and  extended 
missions.  Tasks  also  included  analyzing  data 
from  different  scientific  disciplines  to  provide 
overviews  of  the  scientific  results.  Several  scien- 
tists served  as  chairmen  of  working  groups. 
Scientific  investigations  included  developing 
models  for  the  transport  and  chemistry  of 
hydrogen,  oxygen,  and  carbon  monoxide. 
These  investigations  helped  resolve  questions 
concerning  stability  of  the  carbon  dioxide 
atmosphere,  theory  of  the  atmosphere's  evolu- 
tion, and  formation  of  some  of  its  components 
and  clouds.  T.  M.  Donahue  was  the  scientist 
undertaking  these  tasks.  Another  interdiscipli- 
nary scientist,  D.  M.  Hunten,  coordinated 
preparation  of  a  monograph  on  Venus.  He 
based  his  monograph  on  two  scientific  confer- 
ences. He  also  analyzed  the  voluminous  data 
the  Orbiter  gathered  on  the  neutral  thermo- 
sphere.  He  then  examined  these  data  to  plan 
further  measurements  with  the  Orbiter's 
aeronomy  instruments. 

Siegfried  Bauer  studied,  analyzed,  and  inter- 
preted data  from  Bus  and  Orbiter  experiments. 
His  goal:  to  determine  the  detailed  properties 
of  Venus'  ionosphere  and  its  interactions  with 
solar  wind.  He  accomplished  this  by  investi- 
gating neutral  gas  composition,  thermal  struc- 
ture of  both  neutrals  and  plasma,  and  mass 
transport.  He  also  studied  the  role  of  solar  wind 
and  the  magnetic  field  in  physical  processes 

responsible  for  the  origin,  maintenance,  and 
variability  of  the  planet's  atmosphere. 

Nelson  Spencer  concentrated  on  atmospheric 
motions.  His  research  goals  were  many. 
Among  them  were  assessing  probable  wind- 
vector  parameters  and  calculating  atmospheric 
motions.  He  also  wanted  to  find  out  how  these 
events  correlated  with  other  data  and  how 
they  related  to  basic  questions  about  Venus' 
atmosphere.  To  achieve  his  goals,  Spencer 
analyzed  data  from  the  Orbiter's  neutral 
mass  spectrometer. 

In  a  broad  study  of  radar  data,  G.  H.  Pettengill 
first  analyzed  data  from  the  Orbiter's  onboard 
radar  instrument.  He  then  submitted  his 
abstract  data  for  other  scientists  to  use.  Harold 
Masursky  processed  radar  data  and  correlated 
radar  altimetry.  He  used  image  data  to  produce 
maps  and  Venus  globes.  He  also  used  radar 
data  to  create  topical  studies  of  particular 
Venusian  regions  and  geologic  maps  of  the 
planet's  surface.  By  plotting  radar  altimeter 
data  of  selected  small  regions,  George  E.  McGill 
interpreted  Venus'  topography.  His  efforts 
resulted  in  a  detailed  analysis  of  topography 
and  surface  properties.  He  also  studied  Venus' 
tectonics  and  supported  other  scientists  work- 
ing with  radar  data. 

A.  F.  Nagy  developed  theoretical  models  of  the 
ionosphere  and  performed  comparative  studies 
with  parameterized  models  of  the  planet's 
atmosphere.  He  also  chaired  one  of  the 
working  groups  of  scientists. 

Guest  Investigators 

The  guest  investigator  program  began  in  1981. 
Again,  the  purpose  was  to  involve  new  scien- 
tists in  the  program.  These  scientists  would 
bring  a  fresh  perspective  to  data  analysis  and 
interpretation.  The  guest  investigators  fulfilled 
this  expectation  admirably. 

Since  the  guest  investigators  were  not  necessar- 
ily associated  with  a  specific  instrument,  the 
work  of  only  a  few  appears  here.  Results  of 
their  work  are  in  the  general  science  results  in 
the  next  chapter. 

S.  Kumar,  as  an  example,  investigated  escape  of 
hydrogen  from  Venus.  R.  S.  Wolff  investigated 
the  dayside  ionosphere's  properties  and 
variability  as  a  function  of  solar-wind  condi- 
tions. He  correlated  a  morphological  classifica- 
tion of  ionospheric  density  and  temperature 
profiles  with  several  events.  These  events 
included  solar-wind  dynamic  pressure,  inter- 
planetary magnetic  field  direction,  Sun  zenith 
angle,  and  planetary  latitude.  From  this  classifi- 
cation, he  constructed  a  model  to  show 
ionospheric  dynamics. 

Paul  Rodriguez  analyzed  measurements  of 
plasma  waves  in  the  ionosheath.  He  was  able 
to  derive  the  characteristic  spectrum  of  these 
waves.  From  this,  he  determined  the  impor- 
tant wave-particle  interactions  between  solar 
wind  and  the  ionosphere.  He  compared  these 
with  conditions  in  Earth's  atmosphere  to  gain 
a  new  understanding  of  how  solar  wind 
interacts  with  nonmagnetized  planets. 

Other  guest  investigators  looked  at  many  more 

aspects  of  the  Pioneer  Venus  data:  M.  Dryer 

studied  the  viscous  interaction  of  shocked 

solar  wind  with  Venus'  ionosphere;  J.  C.  Gerard 

examined  chemistry  and  transport  of  thermo-  -,  ~  -, 

spheric  odd  nitrogen;  A.  T.  Young  analyzed 

Venus'  clouds  and  atmosphere;  J.  L.  Fox 

observed  the  role  of  metastable  and  doubly 

ionized  species  in  the  chemical  and  thermal 

structure  of  Venus'  atmosphere  compared  with 

Mars;  S.  S.  Limaye  studied  morphology  and 

movements  of  polarization  features;  and 

C.  O.  Bowin  investigated  Venus'  gravity, 

topography,  and  crustal  evolution. 



In  mid-November  1978,  both  the  Pioneer 
Venus  Orbiter  and  Pioneer  Venus  Multiprobe 
converged  on  their  target.  Venus  had  passed  a 
closest  approach  to  Earth  and  emerged  from 
the  Sun's  glare,  rising  as  a  morning  star  just 
before  the  Sun.  Although  launched  2-1/2  months 
after  Orbiter,  the  Multiprobe  was  catching  up 
with  the  Orbiter.  By  November,  it  was  follow- 
ing closely  behind  it.  The  Orbiter  would  go 
into  orbit  around  Venus  on  December  4.  Five 
days  later,  the  probes  from  the  Multiprobe 
would  make  their  entry  and  hour-long  descent 
through  Venus'  atmosphere.  Mission  control- 
lers prepared  the  Multiprobe  for  the  first  of  its 
four  probes  to  separate. 

The  Interplanetary  Voyage 

There  had  been  dramatic  incidents  during 
the  long  flight  of  the  two  Pioneer  space- 
craft through  interplanetary  space 
(Figure  5-1).  One  incident  occurred  at  the 
time  of  the  Orbiter's  first  significant 
ground-commanded  maneuver  after  it 
left  Earth.  Soon  after  the  spacecraft  was 
launched  on  May  20,  1978,  its  long 
magnetometer  boom  deployed.  The 
dish  antenna  despun  to  face  Earth 
from  the  spinning  spacecraft. 
Mission  controllers  commanded 
checks  of  the  Orbiter  and  several 
of  its  scientific  instruments. 
Telemetry  indicated  all  operated 
according  to  plans.  Next,  they 
tested  the  spin-scan  imaging 
system  by  obtaining  several 
pictures  of  Earth  illuminated 
as  a  thin  crescent. 

To  change  the  velocity  of  the  Orbiter  by 
3.33  m/sec  (7.5  mph),  controllers  commanded 
a  first  in-course  correction  on  June  1,  1978. 
This  maneuver  was  to  aim  the  Orbiter  more 
accurately  at  the  point  near  Venus  where  the 
spacecraft  had  to  fire  its  rocket  motor  to  orbit 
the  planet. 

The  maneuver  did  not  work  out  as  mission 
controllers  planned.  The  cause  turned  out  to 
be  trivial.  It  was  the  first  of  many  operational 
lessons  that  the  project  engineers  controlling 
the  mission  learned  during  the  interplanetary 
voyage.  Engineers  had  designed  the  roll  refer- 
ence system  with  the  safety  feature  of  an  auto- 
matic shut-off.  A  servomechanism  followed 
changes  about  the  roll  axis  at  a  restricted  rate. 
Should  the  spacecraft  change  orientation  too 
quickly,  the  servomechanism  would  lose 
synchronization.  If  this  occurred  during  a 
maneuver,  the  protective  design  halted  the 
maneuver.  In  the  problem  with  the  first 
maneuver,  part  of  the  spacecraft's  structure 
deflected  the  propulsive  jet  from  the  thrusters. 
This  caused  a  propeller-like  action  that 
changed  the  roll  rate  sufficiently  to  drive  the 
servomechanism  too  hard.  As  a  result,  the  first 
maneuver  automatically  aborted.  Once 
controllers  had  identified  the  cause,  they 
successfully  avoided  a  repeat  of  the  problem  by 
issuing  commands  to  disable  the  automatic 
cutoff  circuit  when  it  was  safe  to  do  so. 

The  Pioneer  Mission  to 
explore  Venus  provided  a 
wealth  of  information  on 
Earth's  sister  planet.  The 
mission  far  exceeded  its 
original  aims.  In  its  14  years, 
the  spacecraft  sent  a  continu- 
ous torrent  of  data  from  the 
planet.  This  information 
ranged  from  pictures  of  the 
cloud  cover  to  detailed  radar 
maps  of  Venus' surface.  This 
chapter  details  the  events 
that  occurred  between  late 
1978,  when  the  spacecraft 
converged  with  Venus,  and 
October  1992,  when  the 
Orbiter  completed  its  last 
orbit.  During  those  14  years, 
mission  personnel  gained 
invaluable  experience  in 
space  exploration.  In  this 
chapter,  you  learn  about 
their  challenges,  the 
problems  they  solved,  and 
anomalies  that  remain 


Figure  5- 1 .  Pioneer  Venus 
Orbiter's  path  from  Earth  to 
Venus  carried  it  more  than 
halfway  around  the  Sun  on  its 
seven-month  journey.  At  first, 
the  spacecraft  moved  outside 
Earth 's  orbit,  crossing  inward 
approximately  90  days  after 
launch.  Then  it  moved  toward 
the  Sun  for  a  rendezvous  with 
Venus  in  December  1 978. 


The  necessary  maneuver  was  then  successful, 
but  it  required  8  hours  to  complete  with  a 
series  of  rocket  thrusts  in  two  directions.  The 
spacecraft's  initial  course  carried  it  toward 
Venus'  southern  hemisphere.  The  maneuver 
corrected  the  spacecraft's  path  to  the  required 
orbital  injection  point  some  348  km  (216  miles) 
above  the  planet's  northern  hemisphere.  The 
change  in  flightpath  positioned  the  spacecraft 
so  it  could  achieve  its  planned  elliptical  orbit 
on  arrival  at  Venus.  Science  investigations 

required  an  elliptical  orbit  tilted  75°  to  the 
planet's  equator.  This  would  take  the  space- 
craft to  within  241  km  (150  miles)  of  the 
planet  at  periapsis.  At  apoapsis,  it  would  be 
as  far  away  as  66,000  km  (41,012  miles). 

The  in-course  maneuver  also  slowed  the 
spacecraft,  allowing  it  to  fall  toward  the  Sun. 
Solar  gravity  accelerated  the  spacecraft  so  it 
would  arrive  at  Venus  at  8:00  a.m.  PST  on 
December  4,  1978. 

By  early  June,  the  Orbiter  detected  an 
extremely  powerful  burst  of  gamma  radiation. 
This  was  an  early  and  important  scientific  result 
from  one  of  its  onboard  experiments.  Scientists 
discovered  such  gamma-ray  bursts  in  1973. 
They  possess  enormous  energies  and  occur,  on 
the  average,  about  once  per  month.  Astronomers 
thought  the  bursts  came  from  random  points 
in  the  Galaxy  or  even  from  beyond.  Two  other 
spacecraft  also  observed  this  gamma-ray  burst. 
These  were  Vela,  a  Department  of  Energy 
satellite  circling  Earth,  and  Helios  B,  a  NASA- 
European  spacecraft  orbiting  the  Sun.  By 
triangulation  of  several  such  observations, 
scientists  expected  to  locate  the  bursts'  origins. 
From  these  origins,  they  could  deduce  what 
great  physical  event  might  produce  such 
high-energy  phenomena. 

During  its  voyage  to  Venus,  the  Orbiter  recorded 
a  total  of  six  gamma-ray  bursts.  Two  of  them 
were  among  the  strongest  so  far  recorded.  On 
March  5,  1979,  Orbiter's  instrument  recorded  a 
burst  of  gamma  rays  that,  when  coupled  with 
observations  from  other  spacecraft,  appeared 
to  come  from  the  direction  of  the  Large 
Magellanic  Cloud. 

These  observations  were  important  in  pre- 
liminary investigations  of  such  strange  explo- 
sions in  space.  They  supplemented  later,  more 
detailed  studies  that  used  data  from  the 
Compton  Gamma  Ray  Observatory  (launched 
in  April  1991).  Scientists  could  not  associate 
the  bursts  with  any  object  visible  at  other  wave- 
lengths. They  now  believe  the  bursts  originate 
from  extremely  distant  objects.  Such  objects 
are  far  beyond  the  Magellanic  Cloud,  possibly 
5  to  10  billion  light  years  away.  Later  observa- 
tions showed  that  the  dimmer  high-energy 
gamma  ray  bursts  last  longer  than  the  brighter 
bursts.  This  observation  supports  the  time 
dilation  effects  predicted  by  relativity  theory. 

The  Multiprobe  spacecraft  successfully  com- 
pleted its  first  course  change  on  August  16, 
1978.  Without  a  course  adjustment,  the  Multi- 
probe  would  have  passed  Venus  at  a  distance 
of  about  14,000  km  (8,700  miles)  from  the 
planet's  surface.  This  course  correction 
required  a  day-long  procedure,  featuring  a 
series  of  timed  rocket  thrusts  in  two  directions 
in  space.  It  increased  the  spacecraft's  speed  by 
2.25  m/sec  (about  5  mph). 

There  was  a  minor  incident  during  the  Multi- 
probe's  interplanetary  voyage.  Both  the  Orbiter 
and  the  Multiprobe  carried  redundant  equip- 
ment to  provide  backup  should  a  critical  piece 
of  equipment  fail.  For  example,  the  communi- 
cations system  had  duplicate  power  amplifiers. 
Either  would  work  if  the  other  failed.  There 
were  no  receiver  problems  on  the  Orbiter,  but 
the  command  receivers  were  switched  for  opera- 
tional purposes.  However,  when  engineers 
noticed  a  problem  with  the  Multiprobe's  opera- 
ting receiver,  they  turned  on  the  redundant 
receiver.  Since  the  backup  worked  well,  mis- 
sion controllers  did  not  later  bring  the  original 
receiver  back  into  operation.  Moreover,  the 
Multiprobe  was  fast  approaching  its  rendezvous 
with  Venus  and  needed  many  commands. 

Separation  of  the  Probes 

Splitting  the  Pioneer  Venus  Multiprobe  into  its 
five  independent  spacecraft  provided  two  of 
the  most  crucial  and  exciting  operations  of  the 
Venus  mission.  Rather  small  errors  would  have 
made  the  probes  miss  their  targets  or  fail  on 
entry.  The  Large  Probe  was  scheduled  to  be 
released  on  November  15,  1978.  More  critical 
was  the  scheduled  release  on  November  19  of 
the  three  Small  Probes.  To  reach  the  target  areas 
on  Venus,  the  Small  Probes  had  to  eject  within 
a  few  hours  of  a  preselected  time.  They  had  to 
do  this  within  a  fraction  of  a  degree  in  roll. 



Before  controllers  separated  the  probes,  they 
placed  precisely  calculated  numbers  in  timers 
aboard  each  probe.  These  numbers  represented 
millions  of  seconds  between  release  of  a  probe 
and  the  time  when  its  various  systems  would 
start  operating  for  its  entry  mission.  The 
probes  could  be  released  over  a  period  of  three 
or  four  days.  However,  once  engineers  selected 
a  time,  they  had  to  set  the  timers  precisely  for 
that  time.  Systems  within  each  probe  had  to 
activate  at  the  preestablished  number  of  min- 
utes before  each  probe  entered  Venus'  atmo- 
sphere. "It  was  extremely  critical,"  said  Project 
Manager  Charles  Hall.  "If  the  times  were  set 
short  we  would  have  started  using  the  battery 
(in  each  probe)  too  early  and  run  out  of  power 
by  the  time  we  reached  the  atmosphere.  If  we 
had  set  the  times  too  long,  we  would  have 
missed  a  lot  of  data  as  the  probes  began  to 
enter  the  high  atmosphere." 

The  probes  did  not  accept  uplink  commands 
directly  from  Earth,  only  via  the  Multiprobe. 
As  a  result,  controllers  had  to  set  the  probes' 
timers  before  sending  commands  to  the  Multi- 
probe  to  release  each  probe  from  the  Bus.  They 
had  to  calculate  release  time  from  the  instant 
each  timer  started  counting.  That  counting 
started  when  uplink  commands  turned  on  an 
on-board  clock  pulse.  Activating  commands 
had  to  allow  for  the  one-way  travel  time  of 
signals  from  Earth  to  the  Multiprobe  space- 
craft. That  time  amounted  to  several  minutes. 
To  minimize  human  error  in  those  calculations, 
three  people  derived  them  independently. 

The  Large  Probe  could  not  automatically 
separate  in  the  right  direction  from  the  Multi- 
probe  Bus.  On  November  15,  controllers  had 
to  orient  the  spin  axis  of  the  Bus  so  the  Large 
Probe  would  separate  in  the  right  direction. 
On  the  journey  from  Earth  to  Venus,  they  kept 
the  axis  perpendicular  to  the  ecliptic  plane.  On 
November  9,  commands  moved  it  through 

90°.  This  allowed  the  spacecraft's  medium- 
gain,  aft  horn  antenna  to  communicate  with 
Earth.  The  omnidirectional  antenna  was  not 
suitable  for  Earth  communications  in  checking 
the  probes  before  their  release. 

About  13  million  kilometers  (8  million  miles) 
from  Venus,  controllers  aligned  the  spin  axis 
again.  This  enabled  the  Large  Probe  to  enter 
Venus'  atmosphere  along  a  special  trajectory. 
That  trajectory  would  allow  controllers  to 
orient  the  probe's  heat  shield  correctly  relative 
to  the  entry  flightpath.  However,  when  the 
spin  axis  changed  for  the  Large  Probe's  release, 
tracking  data  from  the  Deep  Space  Network 
(DSN)  were  startling.  Said  Charles  Hall:  "These 
data  did  not  seem  to  add  up  to  what  we  were 
doing  . .  .  there  was  some  question  as  to  the 
precise  direction  the  Bus  was  pointing."  Mis- 
sion controllers  had  to  decide  quickly  whether 
to  command  a  compensating  maneuver. 

Navigating  the  Spacecraft 

A  big  problem  in  determining  orbits  is  measur- 
ing the  north-south  component  of  velocity 
relative  to  Earth.  To  do  this,  navigators  com- 
pare the  difference  in  Doppler  shift  from  a 
tracking  station  in  Earth's  Northern  Hemi- 
sphere with  another  in  the  Southern  Hemi- 
sphere. The  Pioneer  Venus  Multiprobe  needed 
many  maneuvers,  particularly  for  targeting 
entry  points.  First,  controllers  had  to  reorient 
the  antenna  and  spacecraft  to  target  the  Large 
Probe.  Then  they  had  to  reorient  to  release  the 
Small  Probes.  Finally,  they  had  to  reorient  the 
Bus  so  that  it  entered  the  atmosphere  in  a 
special  way.  This  special  orientation  allowed  it 
to  gather  the  maximum  amount  of  data  about 
the  high  atmosphere.  Complicated  bookkeep- 
ing kept  track  of  changes  to  the  spacecraft's 
velocity  vector.  It  also  monitored  how  the 
spacecraft  was  approaching  the  planet.  The 
preseparation  maneuvers  to  release  the  probes 
compromised  the  long  trajectory  tracking 

history  during  the  voyage  from  Earth.  Naviga- 
tors were  concerned  that  they  had  not  mea- 
sured the  orientation  precisely  enough. 
Another  possibility  was  that  the  plume  of  the 
thrusters  had  bounced  off  the  structure  of  the 
spacecraft  and  created  a  sideward  kick. 

When  controllers  were  tracking  the  spacecraft, 
they  were  not  accurately  measuring  its  current 
position  from  an  angular  viewpoint.  Instead, 
they  built  the  trajectory  to  a  current  position 
based  on  the  spacecraft's  previous  positions. 
Traveling  from  Earth  to  Venus,  the  spacecraft 
obeyed  the  laws  of  celestial  mechanics.  It 
moved  along  a  trajectory  calculated  from  those 
laws.  The  tracking  stations  that  observed  it 
were  on  a  rotating  Earth.  Also,  the  Earth  itself 
traveled  in  orbit  around  the  Sun  and  wobbled 
in  concert  with  the  Moon.  To  solve  the  prob- 
lem, navigators  modeled  the  trajectory.  They 
then  compared  their  observations  with  the 
model.  They  continued  to  refine  the  model 
until  the  two  fit. 

Extraneous  effects  that  were  not  in  the  model 
only  began  to  show  up  after  they  had  influ- 
enced the  trajectory  for  some  time.  Navigators 
measured  frequency  shifts  resulting  from  the 
Doppler  effect.  Doppler  residuals  are  the  differ- 
ences between  the  Doppler  shift  according  to 
the  model  and  the  Doppler  shift  in  the  space- 
craft's signal.  Navigators  continually  deter- 
mined, evaluated,  and  used  these  residuals  to 
update  the  model  trajectory.  They  aimed  for 
and  achieved  accuracies  within  a  fraction  of  a 
thousandth  of  a  meter  per  second. 

Before  they  made  any  maneuver,  controllers 
calculated  the  anticipated  Doppler  effect.  If  the 
observed  and  the  expected  Doppler  residuals 
differed  after  the  maneuver,  there  were  two 
possible  explanations.  Either  the  maneuver 
did  not  occur  in  the  planned  direction,  or  the 
thruster  did  not  perform  properly. 

The  Pioneer  Venus  Project  Navigator,  Jack  Dyer, 
explained:  "There  is  a  lot  of  judgment  involved 
in  deciding  on  the  cause.  If  you  know  the  ori- 
entation, the  residual  must  be  due  to  the 
thrusters.  That  is  especially  so  if  the  alignment 
of  the  spin  axis  is,  say,  60°  from  the  direction 
in  which  you  are  observing  the  Doppler  effect. 
It  is  only  when  the  direction  is  perpendicular 
to  the  line  of  sight  from  Earth  that  there  is  an 
unknown  situation."  So  navigators  tried  to  do 
all  maneuvers  in  a  spin-axis  alignment  turned 
somewhat  toward  or  away  from  Earth. 

The  classical  way  to  turn  a  spacecraft  is  to  fire 
two  thrusters  opposite  each  other.  "At  my 
insistence,"  said  Dyer,  "we  fired  only  one 
thruster  to  cause  an  unbalanced  turn,  and 
allowed  the  spacecraft  to  be  propelled.  We  had 
a  very  accurate  means  of  determining  orienta- 
tion of  the  spacecraft  and  had  a  capability  of 
very  precisely  returning  from  one  direction  to 
another  a  few  degrees  away.  These  directions 
could  be  measured  by  the  star  sensors  to  within 
0.01°.  From  such  measurements,  we  could 
calculate  very  accurately  how  much  impulse 
had  been  imparted  to  the  spacecraft  and  there- 
fore how  much  velocity  had  been  applied  in 
the  maneuver."  From  launch,  navigators 
applied  this  unbalance  technique  for  all  space- 
craft maneuvers. 

Pioneer  project  management  considered  one 
possibility  for  the  unexpected  Doppler  data 
from  DSN  after  the  preseparation  maneuver. 
A  propellant  leak  could  have  generated  an 
unwanted  thrust.  This  thrust,  in  turn,  could 
have  pushed  the  spacecraft  from  its  com- 
manded orientation.  Controllers  needed  an 
answer  before  they  could  separate  the  Large 
Probe.  They  scheduled  it  for  release  from  the 
Multiprobe  Bus  at  6:00  p.m.  PST  on  Novem- 
ber 15.  However,  Project  Manager  Charles  Hall 
decided  to  hold  the  release  until  they  could 
identify  the  problem.  "There  were  so  many 



unknowns  at  that  time  that  I  decided  we  had 
better  not  separate  until  we  had  a  better  handle 
on  the  problem.  It  took  us  about  12  hours  to 
see  some  evidence  of  what  the  problem  really 
was.  It  is  amazing  how  these  small  things  take 
so  long  to  sort  out.  It  was  an  all-night  session. 
I  can  recall  that  we  had  a  large  number  of 
engineers  and  scientists  in  the  mission  control 
area.  It  was  too  noisy  to  think,  so  I  brought  a 
cadre  of  top  project  people  into  my  office  and 
we  started  going  over  all  the  calculations.  We 
pieced  the  whole  story  together  until  it  finally 
appeared  that  all  the  diverse  facts  showed  we 
were  on  the  right  track." 

Releasing  the  Large  Probe 

Because  navigators  could  target  the  Large  Probe 
to  enter  the  atmosphere  at  locations  over  a 
large  area  of  Venus,  the  precise  aiming  point 
was  not  critical.  Setting  the  timer,  however, 
was.  Controllers  decided  not  to  attempt 
another  correcting  maneuver.  Rather,  they 
chose  a  timing  setting  that  straddled  the  situa- 
tion. By  contrast,  the  timing  problem  would 
be  serious  with  the  Small  Probes  because  navi- 
gators had  to  target  them  with  extreme  preci- 
sion if  they  were  to  complete  their  missions. 

A  pyrotechnically  released  spring  mechanism 
launched  the  Large  Probe  toward  an  entry  near 
the  equator  on  Venus'  dayside.  Separation  was 
normal.  The  Large  Probe  became  an  indepen- 
dent spacecraft  silently  pursuing  its  path 
toward  the  cloud-shrouded  planet.  Its  internal 
timer  counted  the  seconds  before  its  systems 
had  to  switch  on.  This  would  happen  just 
before  the  probe  encountered  the  rarefied  upper 
regions  of  the  Venusian  atmosphere. 

Targeting  the  Small  Probes 

With  the  Large  Probe  successfully  on  its  path 
to  Venus,  controllers  prepared  to  launch  the 
three  Small  Probes.  During  the  four  days 
before  release  of  the  Small  Probes,  mission 

management  studied  the  Doppler  residual 
uncertainty  problem.  They  recognized  it  was 
probably  an  effect  of  solar  radiation.  The 
problem  occurred  when  they  changed  the 
Multiprobe's  aspect  angle  during  the  pre- 
separation  maneuver.  The  actual  force  of  solar 
radiation  differed  from  what  scientists  had 
modeled  in  the  orbit  determination  program. 
Since  the  spacecraft  had  not  previously 
experienced  this  aspect  angle  and  solar  pres- 
sure modeling  had  otherwise  been  successfully 
treated,  the  discrepancy  came  as  a  surprise. 

One  problem  was  to  achieve  precise  dispersion 
of  the  Small  Probes.  Their  trajectories  did  not 
allow  for  flexibility  in  targeting.  This  was 
especially  so  for  the  probe  that  would  enter 
the  atmosphere  in  the  daylight  hemisphere. 
Careful  judgment  could  prevent  incorrect 
interpretation  of  the  change  in  Doppler  data. 
One  option  was  to  diminish  the  size  of  the 
circle  over  which  the  probes  would  release. 
Navigators  could  achieve  this  by  staying 
inward  of  the  mission's  desirable  boundaries. 

Alignment  of  the  spin  axis  for  release  of  the 
Small  Probes  was  crucial.  Improper  alignment 
could  orient  the  spacecraft  relative  to  the  Sun 
so  its  solar  panels  might  produce  too  little 
power  for  the  Bus  battery.  That  would  have 
limited  the  time  the  battery  could  stay  charged 
at  the  needed  confidence  level.  When  they 
had  reoriented  the  spacecraft,  navigators  had 
to  measure  and,  if  necessary,  adjust  both  the 
attitude  and  the  spin  rate.  They  had  to  release 
the  probes  within  a  period  that  would  not 
deplete  the  battery. 

Before  separation  from  the  Bus,  and  still 
22  days  before  entry,  the  Small  Probes  were 
checked  out  by  radio  command.  All  passed 
their  tests.  Two  days  later,  navigators  reori- 
ented the  Bus.  They  targeted  the  Small  Probes 
to  their  entry  points  (see  Figure  5-2).  One  was 

LP  release 

Bus  retargeting 

SP  release 

1 3:06:29  UT 


02:37:1  BUT 

Dec  9 
11:37:12  UT 

Dec  9 
18:45:32-20:55:34  UT 



Figure  5-2.  In  the  sequence  of 
releasing  the  four  probes,  the 
Large  Probe  was  first.  Next  the 
Small  Probes  separated,  and 
finally  mission  navigators 
retargeted  the  Bus  to  enter  the 
Venusian  atmosphere. 

on  the  dayside  at  midsouthern  latitudes  (the 
Day  Probe).  The  second  was  on  the  nightside, 
also  at  midsouthern  latitudes  (the  Night  Probe). 
The  third  was  on  the  nightside  at  high  north- 
ern latitudes  (the  North  Probe). 

Astronomers  were  aware  of  the  predicted  posi- 
tions for  Venus  before  the  Pioneer  mission. 
Earlier  Venus  flybys  by  Mariner  spacecraft  had 
more  precisely  determined  the  planet's  ephem- 
eris.  Navigators  predicted  that  the  error  in  this 
ephemeris  could  contribute  about  a  30-km 
(18.6-mile)  uncertainty  in  the  direction  of  the 
spacecraft's  arrival  at  Venus.  However,  the 
gravity  of  the  planet  helped;  it  focused  each 
probe  toward  Venus  and  halved  the  uncertainty. 

However,  gravity  did  not  reduce  errors  in  the 
downtrack.  There  the  uncertainty  was  greater, 
amounting  to  hundreds  of  kilometers.  Estimat- 
ing the  downtrack  uncertainty  and  then  plan- 
ning the  encounter  to  this  uncertainty  gave 
navigators  and  mission  planners  a  significant 
problem.  They  had  to  choose  targeting  options 
for  the  five  entry  vehicles.  After  much  discus- 
sion, scientists  and  mission  management 
finally  selected  the  entry  points.  If  the  probes 
entered  at  different  latitudes  and  longitudes 
on  the  planet,  the  mission  would  obtain  the 
best  scientific  data.  The  probes  could  gather 
data  in  day  and  night  hemispheres  and  at 

equatorial  and  high  north  and  south  latitudes. 
There  were,  however,  geometrical  and  commu- 
nications constraints.  The  Bus  spacecraft  com- 
municated to  Earth  from  a  certain  angle 
around  Venus'  hemisphere  from  the  point 
directly  facing  Earth  (the  sub-Earth  point). 
Controllers  had  to  target  the  probes  inward 
from  a  design  boundary  of  communications. 
They  had  to  do  this  by  enough  margin  to 
allow  for  the  estimated  downtrack  uncertainty. 

With  the  Multiprobe  spacecraft  oriented  cor- 
rectly and  spinning  at  about  48  rpm,  clamps 
opened  to  release  the  three  Small  Probes.  They 
left  within  a  millisecond  of  each  other  at  a  pre- 
determined point  in  the  spin  cycle  of  the  Bus. 
The  spin  of  the  spacecraft  and  the  precise  timing 
of  release  directed  the  probes  onto  their  target 
trajectories.  The  timers  in  the  probes  began 
counting  the  seconds  to  atmospheric  entry. 

Mission  of  the  Multiprobe  Bus 

After  all  probes  had  left  the  Bus,  navigators 
maneuvered  it  for  its  own  entry  into  the  atmo- 
sphere. They  slowed  the  Bus  slightly  so  it 
would  reach  Venus  a  short  time  after  the 
probes.  Unlike  the  probes,  the  Bus  did  not 
carry  a  heat  shield  to  protect  it  from  the 
heating  effects  of  high-speed  entry.  Mission 
scientists  expected  it  to  burn  up  within  a  few 
minutes.  However,  during  those  few  minutes, 



its  two  scientific  instruments — ion  and  neutral 
mass  spectrometers — would  gather  data  about 
the  atmospheric  composition.  They  would 
gather  these  data  between  the  140-km 
(87-mile)  and  115-km  (71-mile)  levels. 

One  problem  challenging  navigators  was  how 
to  direct  the  Bus  for  its  entry  into  the  atmo- 
sphere. It  had  to  enter  at  as  shallow  a  flight- 
path  angle  as  possible.  This  angle  would 
reduce  the  heat  load  and  extend  the  period  of 
data  gathering.  However,  at  too  shallow  an 
entry  angle,  the  Bus  could  skip  off  the  top  of 
the  atmosphere.  If  it  did,  it  would  not  get  the 
required  low-altitude  atmospheric  data.  The 
most  desirable  trajectory  would  cause  the  Bus 
to  enter  the  atmosphere,  penetrate  to  the 
115-km  (71 -mile)  level,  and  then  skip  out 
again.  This  would  allow  scientists  to  obtain 
data  along  incoming  and  outgoing  paths. 
Commented  Jack  Dyer,  "We  could  see  that  it 
was  not  possible  to  navigate  so  accurately.  The 
risk  would  be  too  great  that  the  depth  of 
penetration  needed  would  be  missed.  So  we 
decided  to  go  for  as  shallow  an  entry  as  we 
confidently  could." 

Navigators  selected  9°  below  the  local  horizon- 
tal for  the  flightpath  at  200  km  (124  miles) 
above  Venus'  surface.  They  issued  commands 
for  the  spacecraft  to  get  as  close  as  possible  to 
that  path.  Also,  they  set  the  spin  axis  of  the 
Bus  so  the  angle  of  attack  would  be  precisely 
5°.  They  did  this  so  atmospheric  molecules 
would  enter  the  scientific  instruments  prop- 
erly. After  navigators  had  completed  these 
maneuvers,  all  the  probes  and  the  Bus  were 
on  their  way  to  their  targets. 

Arrival  of  the  Orbiter 

Meanwhile,  Pioneer  Orbiter  approached  its 
rendezvous  with  Venus.  Controllers  would 
maneuver  the  spacecraft  into  orbit  before  the 
probes  arrived  at  the  planet. 

December  4  was  the  date  the  mission  selected 
for  the  speeding  Orbiter  to  slow  into  an 
elliptical  path  around  Venus  (Figure  5-3).  The 
maneuver  had  to  take  place  behind  Venus  as 
viewed  from  Earth,  and  this  worried  control- 
lers. The  spacecraft  was  out  of  communication 
for  almost  23  minutes  at  this  extremely  critical 
milestone.  During  this  essential  maneuver,  a 
180-kg  (400-lb)  solid-propellant  rocket  motor 
fired.  It  slowed  the  Orbiter  sufficiently  for 
Venus'  gravity  to  capture  the  spacecraft  into 
orbit  around  the  planet.  This  event  was  the 
first  time  a  solid-propellant  rocket  had  been 
fired  after  being  in  space  for  seven  months — 
the  time  between  the  launch  from  Earth  and 
arrival  at  Venus. 

On  December  2,  the  Orbiter  started  maneuvers 
for  its  insertion.  It  began  with  an  orientation 
to  point  the  rocket  nozzle  in  the  direction  of 
travel.  Controllers  lowered  the  communications 
bit  rate  from  1024  to  64  bits/sec.  This  allowed 
the  omnidirectional  low-gain  antenna  to 
maintain  communications  during  the  reorien- 
tation  maneuver  instead  of  the  high-gain 
antenna.  Next,  the  high-gain  antenna  was 
released  and  spun  up  to  match  the  spacecraft's 
spin  rate.  The  spin  rate  increased  to  30  rpm. 
Next  the  high-gain  antenna  was  despun,  and 
the  bit  rate  returned  to  1024  bits/sec. 

The  Orbiter's  flight  from  Earth  had  been  free 
of  major  problems.  However,  there  had  been 
minor  problems  in  the  command  memories 
on  the  way  to  Venus.  These  problems  could 
have  led  to  serious  difficulties  in  obtaining  a 
correct  injection  into  orbit.  High-energy  solar 
cosmic  rays  had  caused  "bit-flip"  errors  in  the 
spacecraft's  memories.  They  had  changed  ones 
to  zeros  and  vice  versa.  These  errors  occurred 
on  an  average  of  about  once  every  two  weeks. 
They  could  have  resulted  in  a  command 
sequence  being  interrupted  or  changed.  Fortu- 
nately, when  these  bit-flips  occurred, 

controllers  could  correct  them  or  the  com- 
mand had  already  been  executed.  However,  if 
such  an  error  occurred  in  the  command  timing 
sequence  for  the  rocket  motor,  it  might  have 
caused  premature  or  delayed  rocket  firing  for 
the  orbital  insertion  maneuver.  The  results 
would  have  been  disastrous. 

Bit-flip  errors  occurred  on  both  the  Orbiter 
and  the  Multiprobe  in  transit  to  Venus.  The 
problem  surfaced  so  late  in  the  Pioneer  Venus 
program,  design  changes  to  overcome  it 
were  not  practical.  Although  the  bit-flips 
occurred  on  the  Orbiter  in  flight  before  the 
Multiprobe  was  launched,  it  was  much  too  late 
to  make  design  changes  for  the  Multiprobe. 
Fortunately,  they  were  not  as  critical  for  the 
Multiprobe's  operation. 

Bit-flips  had  probably  affected  interplanetary 
spacecraft  before.  Scientists  had  to  be  able  to 
compare  what  went  into  a  spacecraft's 
memory  with  what  came  out  of  it.  Only  then 
could  they  clearly  identify  such  events.  Until 
Pioneer  Venus,  there  had  been  no  oppor- 
tunity during  a  mission  to  check  spacecraft 
memories  for  these  bit-flip  effects.  Actually, 
bit-flips  had  been  discovered  on  some  Earth- 
orbiting  satellites.  Ironically,  they  resulted 
from  the  same  high  technology  that  can 
minimize  energy  to  flip  a  digital  circuit  from 
one  state  to  the  other.  A  high-energy  cosmic 
ray  particle  could  provide  sufficient  energy. 

To  overcome  these  bit-flips  on  Pioneer,  con- 
trollers took  great  care  in  how  they  stored  com- 
mands in  the  command  logic.  Before  execution, 
they  always  checked  commands  that  they  had 
stored  for  any  period.  This  procedure  ensured 
that  nothing  had  changed  in  the  commands. 

A  bit-flip  could  have  serious  consequences 
during  the  Orbiter's  injection  maneuver.  This 
was  particularly  true  if  it  changed  the  timing 



at  time  of 



sequence  to  ignite  the  motor.  This  sequence 
had  to  start  while  the  spacecraft  was  in  radio 
communication  with  Earth.  Also  it  had  to  start 
before  the  spacecraft  went  behind  Venus. 

A  bit-flip  could  change  a  time  delay  that  pro- 
grammers put  into  the  spacecraft's  memory  to 
control  the  motor's  ignition.  Such  a  change 
was  unacceptable.  Alternatively,  a  sequence  of 
small  time  delays,  whose  sum  would  be  the 
total  time,  could  command  the  ignition  count- 
down. Analysis  showed  that  greatest  reliability 
would  result  from  a  series  of  time  delays  in 
two  redundant  command  memories.  Should  a 
bit-flip  affect  the  time  delay  in  either  parallel 
memory,  it  would  have  had  no  ill  effect.  By 
contrast,  a  jump  to  early  rocket  firing  by  either 
memory  alone  would  have  been  disastrous. 

Figure  5-3.  This  perspective  of 
the  orbit  around  Venus  identifies 
the  orbital  insertion  point,  the 
periapsis,  and  the  apoapsis 
relative  to  the  planet.  The  hour 
marks  along  the  orbit  show  how 
the  Orbiter  traveled  quickly 
through  periapsis  and  more 
slowly  around  apoapsis. 



On  December  3,  at  11:00  p.m.  PST,  controllers 
loaded  the  Orbiter's  two  command  memories 
with  the  command  sequence  for  firing  the  orbit 
insertion  motor.  The  firing  would  occur  at 
7:58  a.m.  PST  on  December  4  (Figure  5-4).  Of 
over  40  command  delays,  the  first  few  were  for 
1  hour,  the  next  for  45  minutes,  then  30  min- 
utes, then  delays  of  1  minute,  then  another 
batch  of  3  seconds  each.  The  command  mem- 
ory countdown  started  at  1:00  a.m.  PST  on 
December  4.  Each  time  the  memory  counted 
out  one  of  the  delays  without  error,  the  space- 
craft signaled  the  successful  timing  execution. 

Insertion  into  Orbit 

At  7:51  a.m.  PST  on  December  4,  the  Orbiter 
passed  behind  Venus,  and  communications 
with  Earth  were  interrupted.  If  all  went  well, 
the  orbit  insertion  commands  in  the  space- 
craft's memory  would  fire  the  rocket  motor 
7  minutes  later.  The  motor's  propellant  would 
burn  for  almost  30  seconds  and  change  the 
spacecraft's  velocity  by  about  3780  km/hr 
(2349  mph). 

Controllers  had  set  the  spacecraft  orientation 
and  the  altitude  of  the  closest  approach  of  the 
flyby  trajectory.  They  had  timed  the  firing  of 
the  retrorocket  precisely.  It  would  thrust  the 
Orbiter  into  an  orbit  as  near  as  possible  to  the 
mission's  nominal  orbit.  Controllers  would 
have  to  correct  later  any  errors  made  in  timing 
the  firing  of  the  retrorocket.  They  also  would 
have  to  wait  to  correct  any  error  in  the  total 
impulse  developed  by  the  rocket  motor.  Since 
corrections  would  need  propellant  and  would 
reduce  the  reserve  for  maneuvering  in  orbit, 
they  would  be  undesirable.  This  would  shorten 
the  time  during  which  navigators  could 
control  the  Orbiter's  periapsis  altitude  to 
obtain  upper  atmospheric  science  data. 

As  the  spacecraft  approached  Venus,  it  had  a 
good  propellant  reserve  because  the  launch 

had  been  early  in  the  launch  opportunity. 
Mission  scientists  wanted  to  preserve  the  cap- 
ability of  maintaining  orbit  for  one  Venusian 
sidereal  day.  (This  was  the  mission  design 
capability.)  As  a  result,  they  made  no  attempt 
initially  to  stretch  the  mission  to  ultimate 
design  requirements.  A  Venusian  sidereal  day 
is  different  from  a  Venusian  solar  day.  The 
sidereal  day  is  the  planet's  rotation  period 
relative  to  inertial  space.  A  solar  day  is  the 
rotation  period  relative  to  the  Sun.  The  Venu- 
sian sidereal  day  is  243.1  Earth  days.  The  solar 
day  is  116.8  Earth  days. 

Maintaining  propellant  reserves  was  important 
because  there  were  data  transmission  limits  to 
the  mission.  Experiments  could  gather  more 
data  than  the  radio  link  to  Earth  could  handle. 
It  was  a  foregone  conclusion  that  experiment- 
ers would  want  the  spacecraft  to  continue  in 
orbit  after  the  first  sidereal  day.  Their  goal  was 
to  gather  and  transmit  data  into  a  second 
sidereal  day.  In  such  an  extended  mission, 
investigators  would  change  emphasis  on  the 
types  of  data  they  gathered  and  transmitted. 
To  preserve  this  capability,  controllers  had  to 
budget  propellant  usage  and  conserve  reserves. 

Getting  the  spacecraft  into  orbit  was  exciting 
for  project  management,  said  Charles  Hall. 
"We  had  never  done  anything  like  this  before. 
Ignition  of  the  rocket  motor  behind  the  planet 
meant  there  was  always  the  question  of 
whether  or  not  the  motor  had  ignited."  To 
ensure  that  the  spacecraft  got  into  orbit, 
controllers  sent  a  second  ignition  command. 
They  timed  it  to  arrive  at  the  spacecraft  after 
its  emergence  from  behind  Venus.  This  backup 
command  would  start  ignition  if  the  earlier 
command  had  not  worked  behind  the  planet. 
The  orbit  would  not,  of  course,  have  been  as 
good  from  such  a  late  ignition.  But  it  would 
have  prevented  the  spacecraft  from  flying  past 
Venus  and  going  into  solar  orbit. 

Dec  2:  Orient  spacecraft  for  orbit 
insertion  and  spin-up  to  30  rpm 

planet  — 
no  radio 



Solid  rocket 

motor  burns 

30  sec, 



3780  km/hr 

(2349  mph) 

11  p.m.  Dec  3:  Load  command 

memories  to  fire  orbit 

insertion  motor 

1  a.m.  Dec  4:  Start 


memories  to 

fire  orbit 

insertion  motor 

trim  to 
5  rpm 

to  high- 

to  south 

30  to 
15  rpm 




Hall  explained  how  ignition  was  confirmed. 
"If  we  had  ignition,  then,  when  the  spacecraft 
emerged,  the  frequency  of  the  carrier  radio 
wave  (from  the  spacecraft)  would  be  different 
from  that  if  ignition  had  not  occurred  (because 
of  Doppler  effects).  I  recall  that  we  had  two 
receivers  on  the  ground  waiting  to  pick  up 
signals  on  one  or  the  other  frequency.  At 
8:14  a.m.  PST,  the  spacecraft  emerged  from 
behind  Venus.  It  took  3  minutes  for  the  radio 
signals  to  travel  the  56  million  km  (35  million 
miles)  to  Earth.  Everyone  waited  for  one  of  the 
two  ground  receivers  to  lock  onto  the  space- 
craft's signal.  When  it  was  clear  that  the  right 

receiver  had  locked  onto  the  signal  from 
Pioneer  Orbiter,  there  was  a  big  cheer  because 
we  knew  then  that  the  spacecraft  had  gone 
into  orbit." 

At  8:30  a.m.  PST,  navigators  adjusted  the 
Orbiter's  spin  rate  to  15  rpm.  The  high-gain 
antenna  despun  and  pointed  toward  Earth. 
Within  the  next  few  hours,  navigators  ana- 
lyzed tracking  data  to  determine  the  param- 
eters of  the  orbit  around  Venus.  The  highly 
elliptical  orbit,  inclined  75°  to  the  equator  of 
Venus  (105°  retrograde)  was  almost,  but  not 
quite,  as  they  expected.  Table  5-1  gives  the 

Figure  5-4.  Operations  of  the 
Orbiter  spacecraft  before, 
during,  and  after  the  period 
of  insertion  into  orbit. 


Table  5-1.  Planned  and  Initial  Orbit  Parameters 




Periapsis  altitude,  km  (miles) 
Periapsis  latitude,  deg 
Periapsis  longitude,  deg 
Inclination,  deg 
Period,  hr:min:sec 

203.223  E 

207.990  E 


orbit  parameters  the  injection  burn  achieved, 
compared  with  those  the  mission  planned. 

Navigator  Jack  Dyer  explained  the  problems 
of  entering  an  orbit  around  another  planet. 
"We  had  to  be  very  precise  with  navigation  so 
that  the  burning  of  a  given  weight  of  propel- 
lant  would  put  the  spacecraft  into  orbit.  We 
spent  a  lot  of  time  determining  how  accurately 
we  thought  the  manufacturer  of  the  retro- 
rocket  could  predict  the  amount  of  impulse  it 
would  deliver." 

The  retrorocket  performed  better  than  naviga- 
tors predicted.  This  was  as  bad  as  underper- 
forming.  The  over-performance  had  slowed 
the  Orbiter  too  much  and  resulted  in  the  apo- 
apsis  being  lower  than  they  had  planned.  The 
periapsis  was  higher,  too.  It  also  resulted  in  a 
shorter  orbital  period  of  23  hours  1 1  minutes. 
As  a  result,  navigators  had  to  use  more  propel- 
lant  from  the  attitude  control  subsystem  to 
correct  the  orbit's  period  to  the  required 
24  hours. 

Mission  navigators  had  to  adjust  the  period  at 
periapsis.  Because  the  first  orbits  were  times  of 
great  scientific  activity,  they  had  to  delay  the 
adjustment  for  two  orbits.  In  the  meantime, 
however,  they  began  a  preplanned  maneuver 
to  lower  the  periapsis  from  378  km  (234  miles) 
to  250  km  (155  miles).  This  took  place  at 
apoapsis  on  December  5  by  firing  two  of  the 
spacecraft's  thrusters  for  slightly  longer  than 
3  minutes. 

Initial  orbital  operations  followed  a  carefully 
preplanned  sequence  (Figure  5-5).  At  3:00  p.m. 
PST  on  December  4,  commands  reduced  the 
spin  rate  to  6  rpm  from  15  rpm.  Others 
adjusted  the  spin  axis  to  point  toward  the 
celestial  poles.  Then,  a  couple  of  hours  later, 
controllers  pointed  the  high-gain  antenna 
toward  Earth.  Communications  switched  to  it 
from  the  omni-antenna.  In  the  following  hours, 
scientists  activated  some  of  the  scientific  instru- 
ments. The  first  were  the  spectrometer  and 
the  electron  temperature  probe.  Next,  they 
unlocked  the  radar  antenna  and  deployed  the 
boom  of  the  electron  temperature  probe.  The 
neutral  mass  spectrometer  and  the  radar  went 
through  calibration  sequences.  Just  after  the 
first  orbit  began,  at  the  first  apoapsis  after  the 
spacecraft  entered  orbit,  controllers  activated 
the  magnetometer  and  the  retarding  potential 
analyzer.  The  first  orbital  data-gathering 
sequence  had  started.  The  remaining  instru- 
ments were  turned  on  later  in  that  first  orbit  at 
times  requested  by  the  principal  investigators. 

The  short  orbital  period  caused  the  time  of 
periapsis  to  occur  earlier  each  Earth  day  than 
desired.  As  a  result,  it  affected  assignment  of 
tracking  stations.  Mission  controllers  had  to 
rearrange  the  relative  geometries  of  the  space- 
craft, of  Venus,  and  of  Earth's  tracking  stations 
so  key  tracking  stations  at  Goldstone,  Califor- 
nia, and  Canberra,  Australia,  could  receive 
signals  from  the  spacecraft  at  a  preselected  part 
of  its  daily  orbit  around  Venus.  After  two  orbits 
of  the  spacecraft,  navigators  fired  thrusters  at 
periapsis  on  December  6.  This  increased  the 

planet  — 
no  radio 







30  sec, 



3780  km/hr 

(2349  mph) 


Magnetometer  and  retarding  potential  analyzer  on 

30  to 
15  rpm 




Orbit  determination 

Load  radar  mapper 
memory  for  first  orbit 

End  orbit 



on;  unlock 




neutral  mass 


(NMS)  hat; 




probe  (ETP) 

boom;  NMS 

and  ETP  on 

to  high- 

to  south 

Figure  5-5.  The 
orbit  insertion 
sequence  ended 
1 5  hours  after 
achieving  orbit. 
The  first  opera- 
tional orbit  began 
after  controllers 
turned  on  several 
instruments  just 
before  the  first 
apoapsis  passage. 
These  highlights 
are  called  out  in 
this  schematic 

orbital  period  to  just  over  24  hours.  Afterward, 
the  time  of  periapsis  gradually  moved  within 
an  acceptable  range. 

Once  the  24-hour  orbit  was  achieved,  mission 
operations  divided  it  into  two  segments.  Each 
segment  reflected  the  kind  of  measurements 
they  were  taking  (Figure  5-6).  The  periapsis  seg- 
ment was  about  4  hours  long.  The  apoapsis 
segment  was  20  hours  long.  Mission  opera- 

tions used  one  of  five  data  formats  during  each 
short  periapsis  segment.  The  formats  made  it 
possible  to  emphasize  certain  experiments 
when  desirable.  For  example,  one  format  was 
for  intensive  aeronomy  coverage  at  periapsis 
and  another  was  for  optical  coverage. 

Normally,  scientists  used  only  one  of  two  data 
formats  in  the  20-hour  apoapsis  segment  of 
the  daily  orbit.  The  first  was  for  obtaining 


Figure  5-6.  This  orbit  schematic 
illustrates  how  operations 
proceeded  through  the  mission 
on  a  regular  basis.  Sequences 
were  modified  at  times  to 
satisfy  requirements  of 
experimenters  and  periods  of 
occultations  and  eclipses. 

Start  stored  commands  to 

operate  ion  mass  spectrometer 

(IMS),  ultraviolet  spectrometer 

(UVS),  retarding  potential 

analyzer  (RPA),  neutral  mass 

spectrometer  (NMS),  infrared 

radiometer  (IR),  radar  mapper, 

plasma  analyzer  (PA), 

electron  temperature  probe 

(ETP)  during  occupation 



memory  load 




1  orbit  =  24  hr 

Gamma-burst  detector,  magnetometer 

and  electric-field  detector  operate 

continuously  during  orbit 

End  stored 








Load  radar 



Cloud  photopolarimeter 
observations  and  plasma 
analyzer  sequences 

NMS,  IR, 

and  PA 


RPA  and  PA  sequences 

Readout  data 
storage  units 


images  of  the  planet's  whole  disk  in  ultraviolet 
light  to  record  cloud  features  (Figure  5-7).  It 
allocated  67%  of  the  data  stream  to  imaging 
data.  It  divided  the  balance  of  the  data 
transmission  among  the  three  instruments 
that  measured  solar-wind  and  planet  interac- 
tions and  the  gamma-burst  detector.  The  other 
format  allocated  data  return  to  all  instruments 
except  the  imaging  instrument  and  the 
infrared  radiometer. 

By  December  6,  NASA  had  successfully 
received  the  first  image  of  Venus  (Figure  5-8), 
and  science  data  were  flowing  to  Earth.  All  was 
going  well  with  the  Orbiter  spacecraft. 

Entry  of  the  Probes 

When  the  probes  separated  from  the 
Multiprobe  Bus,  they  went  "off  the  air."  This 
happened  because  they  did  not  have  sufficient 
on-board  power  or  solar  cells  to  replenish  their 

batteries.  There  was  no  way  to  command  the 
probes  from  Earth.  Preprogrammed  instruc- 
tions were  wired  into  them,  and  their  timers 
had  been  set  before  they  separated  from  the 
Bus.  The  on-board  countdown  timers  were 
scheduled  to  bring  each  probe  into  operation 
again.  This  would  occur  3  hours  before  they 
began  their  descent  through  the  Venusian 
atmosphere.  This  was  timed  for  7:50  a.m.  PST 
on  December  9,  1978.  The  timers  had  to  turn 
on  heaters  to  warm  the  battery  and  the  stable 
oscillators  of  the  radio  transmitters.  This 
ensured  that  the  carrier  frequencies  would  be 
correct  when  the  transmitters  began  sending 
signals  to  Earth  shortly  before  entry.  Later,  the 
command  unit  started  warmup  and  calibration 
cycles  for  the  three  instruments  on  each  probe. 

At  8:15  a.m.  PST,  the  command  timer  on  the 
Large  Probe  began  warmup  of  the  Probe's 
battery  and  radio  receiver.  The  latter  received  a 





Hours  from 


Figure  5-7.  Cloud  photo- 
polarimetry  used  motion  along 
the  Orbiter's  flightpath  and 
rotation  of  the  spacecraft  to 
scan  the  planet  in  ultraviolet 
radiation.  The  instrument  could 
make  five  planetary  images  in 
each  orbit  with  a  resolution  of 
about  30  km  (19  miles).  The 
instrument  determined  cloud 
particle  characteristics  from 
polarization  measurements, 
made  images  of  haze  layers  at 
the  planet's  limb  with  a 
resolution  of  15  km  (9.3  miles), 
and  observed  several  comets. 

carrier  frequency  from  Earth  to  spacecraft  that 
provided  the  reference  frequency  for  the 
downlink  signal  from  spacecraft  to  Earth. 

At  10:23  a.m.  PST,  the  Large  Probe  began  to 
transmit  radio  signals  to  Earth  for  two-way 
Doppler  tracking  at  256  bits/sec.  This  occurred 
just  22  minutes  before  entry.  The  22-minute 
interval  was  a  compromise  between  consum- 
ing precious  battery  power  and  providing  DSN 
stations  with  sufficient  time  to  lock  onto  the 
signals  before  the  probes  began  to  send  entry 
data.  Within  the  next  11  minutes  after  the 
Large  Probe's  transmission  began,  all  the  Small 
Probes  started  transmitting.  First  came  the 
signal  from  the  North  Probe,  then  the  Day 
Probe,  and  finally  the  Night  Probe. 

Seventeen  minutes  before  hurtling  into  the 
Venusian  atmosphere  at  42,000  km/hr 
(26,099  mph),  each  Small  Probe  began  trans- 
mitting data  at  a  rate  of  64  bits/sec.  The  Large 
Probe  transmitted  at  256  bits/sec. 

Charles  Hall  related  how,  several  months 
before  the  encounter  with  Venus,  a  group  from 

the  Pioneer  project  traveled  into  California's 
Mojave  Desert.  The  purpose  of  the  trip  was  to 
visit  DSN's  isolated  Goldstone  Tracking 
Station.  There  the  group  reviewed  the  station's 
equipment  and  operating  procedures  for 
obtaining  data  from  the  probes  during  their 
entry  into  Venus'  atmosphere.  The  operators 
at  Goldstone  went  through  encounter  simula- 
tions to  demonstrate  how  the  actual  mission 
would  occur.  The  aim  was  to  identify  and 
eliminate  potential  operational  and  ground 
equipment  problems. 

Operators  simulated  the  five  frequencies  from 
the  four  probes  and  the  Bus.  This  simulation 
represented  the  expected  form  of  the  frequen- 
cies when  they  arrived  from  the  distant  space- 
craft fleet  as  it  approached  Venus.  Equipment 
received  radio  signals  from  these  spacecraft  in 
an  open-loop  mode.  That  is,  reception  occurred 
without  using  the  output  to  correct  the  input. 
If  the  frequency  of  a  carrier  emitted  by  any 
spacecraft  were  detected,  a  small  blip  would 
appear  among  radio  noise  on  a  monitor  screen. 
"When  I  first  saw  this  screen  and  the  blip,  it 
looked  like  a  rowboat  in  the  middle  of  the 


Figure  5-8.  (Left)  The  first  image 
of  Venus  from  the  Pioneer 
Orbiter  reached  Earth  on 
December  6,  1 978.  It  showed 
the  planet  in  a  crescent  phase. 
(Right)  Subsequent  images  at 
increasing  phases  showed  much 
greater  detail  of  the  Venusian 
cloud  systems.  This  image  was 
received  on  December  25,  1 978. 


Atlantic  Ocean  during  a  storm,"  said  Hall.  "We 
could  hardly  see  the  blip  for  all  the  noise.  A 
crowd  of  dots  moved  up  and  down  on  the 
screen  and  only  one  of  them  was  still.  Highly 
skilled  operators  had  to  be  very  alert  to  see  the 
stationary  blip." 

He  recounted  how  the  operators  became  very 
skilled  in  finding  the  blip  among  the  noise. 
"They  homed  in  on  it  by  reducing  the  band- 
width so  that  the  blip  stood  out  clearly  from 
the  noise,  bringing  a  pointer  to  the  correct 
frequency  and  pressing  a  button.  This  started 
an  automatic  calculation  so  that  the  operator 
of  the  closed-loop  receiver  could  have  infor- 
mation to  set  into  his  control  dials  and  get  the 
real-time  data  flowing  from  the  simulated 
probes.  In  this  way,  the  operators  were  able  to 
change  to  a  closed-loop  system  and  lock  onto 
a  simulated  signal  within  seconds." 

These  extensive  practice  runs  paid  off  when 
the  probes  reached  Venus.  During  the  encoun- 
ter, friendly  competition  developed  between 
the  two  tracking  stations  at  Goldstone  and 
Canberra.  Which  station  would  be  first  to 
detect  the  radio  signals  when  the  probes 

entered  the  atmosphere?  Said  Hall,  "I  guess 
the  most  exciting  part  of  the  mission  was  to 
hear  the  DSN  (audio  communications)  as  the 
probes  were  turned  on  and  their  signals  were 
received  and  locked  onto." 

The  first  signal  came  from  the  Large  Probe.  It 
left  the  probe  at  10:24  a.m.  PST  on  December  9 
and  arrived  at  Earth  3  minutes  later. 

Said  Hall;  "When  we  got  the  message — 'We've 
locked  up  on  the  Large  Probe' — everyone 
cheered.  Then  three  or  four  minutes  later,  we 
heard  'Forty-three  (ID  for  the  Canberra  station) 
has  locked  up  on  a  Small  Probe/  and  so  on, 
right  down  the  line.  First  one  station  and  then 
the  other  announced  a  lockup.  In  retrospect,  it 
was  a  tie  between  the  stations." 

One  by  one,  and  within  a  few  minutes,  each 
probe  reestablished  communications  with  the 
Pioneer  Mission  Operations  Center  (PMOC)  at 
Ames  Research  Center  in  California.  Shortly 
after  each  probe  had  been  acquired,  it  was 
sending  data  to  Earth.  By  10:45  a.m.  PST,  the 
Operations  Center  reported  that  all  instru- 
ments were  operating  satisfactorily. 

"We  had  been  waiting  for  24  days  (for  the 
Large  Probe)  and  for  19  days  (for  the  Small 
Probes).  To  have  them  come  on  within  a  split 
second  of  the  times  they  were  supposed  to, 
and  particularly  to  have  the  ground  stations 
lockup,  was  quite  an  achievement,"  com- 
mented Hall.  "I  think  that  the  lockup  of  the 
four  probes  was  probably  one  of  the  most  dif- 
ficult tasks  that  the  DSN  has  ever  had  to 
deal  with." 

Five  minutes  before  each  Small  Probe  entered 
the  atmosphere,  it  deployed  the  two  cables 
and  weights  of  its  yo-yo  despin  system.  These 
enabled  it  to  reduce  its  spin  rate  from  48  to 
15  rpm.  The  Bus  imparted  high  spin  rates  to 
disperse  the  probes  to  entry  points  widely 
spaced  over  the  planet.  However,  this  wide 
dispersion  had  another  consequence.  It  meant 
that  the  smaller  probes  entered  the  Venusian 
atmosphere  somewhat  tilted  off  their  flight- 
paths.  The  spindown  of  the  probes  allowed 
aerodynamic  forces  to  line  up  their  axes  with 
the  desired  flightpaths.  This  had  to  occur 
quickly  before  heating  at  the  edges  of  a  probe's 
conical  heat  shield  could  become  serious.  The 
probes  jettisoned  the  cables  and  weights 
immediately  after  spindown. 

At  200  km  (124  miles)  above  the  surface  of 
the  planet,  the  probes  plunged  into  the  atmo- 
sphere at  almost  42,000  km/hr  (26,099  mph). 
Expected  entry  communications  blackout 
occurred  as  the  heated  atmosphere  flowing 
around  the  heat  shield  ionized.  The  plasma 
blocked  the  communications  signal  for  about 
10  seconds.  After  this  blackout,  the  probes 
were  moving  more  slowly.  Now  the  tracking 
stations  had  to  reacquire  their  signals  at  a 
different  radio  frequency.  The  DSN  success- 
fully locked  again  on  all  the  probes'  signals 
after  each  went  through  its  individual 
radio  blackout. 

Now  the  most  exciting  part  of  the  mission 
began.  Enormous  pressure  and  intense  heat 
coupled  with  acid  chemical  corrosion  in 
Venus'  atmosphere  were  the  great  environ- 
mental challenges  to  engineers  responsible  for 
designing  and  building  the  probes.  For 
example,  the  Large  Probe  had  to  jettison  its 
parachute  to  speed  its  descent  through  the 
thick,  lower  atmosphere.  In  this  way,  the 
probe  could  telemeter  data  all  the  way  down 
to  Venus'  surface.  A  slower  descent  would  have 
heated  the  probe  to  dangerously  high  tempera- 
tures before  it  reached  the  lower  atmosphere. 
This  would  have  prevented  it  from  obtaining 
information  there. 

An  earlier  chapter  recounted  how  the  probe 
pressure  vessels  were  constructed  from  tita- 
nium. Titanium  is  a  light  but  strong  metal  that 
is  very  difficult  to  machine.  Deep  in  Venus' 
atmosphere,  the  probes  would  encounter 
enormous  pressures.  To  withstand  these 
pressures,  designers  applied  experience  from 
building  bathyspheres  for  exploring  Earth's 
deep  oceans. 

Each  pressure  vessel  needed  multiple  ports  so 
scientific  instruments  could  access  the  ambi- 
ent atmosphere.  There  were  19  such  penetra- 
tions in  the  Large  Probe's  pressure  vessel  and 
7  in  each  Small  Probe.  Protecting  the  vessels 
against  the  great  range  of  outside  pressures  had 
presented  many  engineering  difficulties.  Seal- 
ing windows  against  pressure  and  heat  was 
perhaps  the  most  demanding  task.  For  example, 
the  sapphire  windows  often  cracked  when  engi- 
neers tested  them  at  high  temperature.  As  a 
result,  designers  thickened  them  so  they  could 
survive  the  conditions  on  Venus.  A  brazed  seal 
for  use  with  the  diamond  windows  had  deteri- 
orated when  tested,  too.  Engineers  replaced  it 
with  complex  seals  of  Graphoil,  Anviloy 
(containing  90%  tungsten),  and  Inconel. 


Figure  5-9.  This  ground- 
based  picture  of  Venus  was 
taken  by  Jay  Apt  with  the 
60-in.  Mt.  Hopkins  Observatory 
telescope,  Tucson,  Arizona.  He 
took  it  when  the  probes  plunged 
into  the  Venusian  atmosphere. 
The  image  was  obtained  at  a 
wavelength  of  11 .5  micrometers. 
Circles  show  the  entry  points  of 
the  Small  Probes.  A  triangle 
shows  the  Large  Probe's  entry 


As  the  probes  plunged  toward  Venus,  engineers 
anxiously  awaited  results  that  would  confirm 
the  success  of  their  designs.  Although  the 
probes  had  withstood  rigorous  tests  before 
launch,  there  was  always  the  possibility  that 
Venus'  environment  could  hold  some  surprises. 

The  probes  were  protected  in  several  ways 
against  heat  arising  from  their  high-speed 
entry  into  the  atmosphere  and  from  the  high 
ambient  temperature  deep  in  that  atmosphere. 
Heat  shields,  chiefly  of  carbon  phenolic, 
protected  the  probes  against  excessive  heating. 
Transfer  of  entry  heat  to  the  scientific  instru- 
ments was  controlled  by  mounting  the 
instruments  on  heat  absorbers  (sinks).  These 
consisted  of  beryllium  shelves  for  the  Large 
Probe  and  aluminum  shelves  for  the  Small 
Probes.  Multilayered  protective  blankets  of 
plastic  sheet  that  were  extremely  heat  resistant 
further  limited  heat  transfer.  Filling  the 
probe's  interior  with  the  inert  gas  xenon 
reduced  conduction  of  heat  through  the 

atmosphere  inside  the 
Small  Probes.  This  gas 
conducts  only  about 
21%  the  amount  of 
heat  that  air  does.  The 
aim  was  to  keep  each 
probe's  interior  below 
50°C  (122°F)  in  an 
ambient  environment 
with  temperatures  as 
high  as  493°C  (920°F). 

As  the  time  for  entry 
approached,  excite- 
ment rose  dramati- 
cally. This  was  par- 
ticularly true  at  the 
PMOC  and  at  the 
many  contractors' 
plants  that  helped 
design  the  Pioneer 

Venus  vehicles.  Many  years  of  design  and 
exhaustive  ground-based  simulations  were 
about  to  be  put  to  their  ultimate  test.  Everyone 
waited  as  the  four  probes  plowed  through  the 
global  haze  and  sulfuric  acid  clouds,  through 
the  violent  winds,  and  the  hot  carbon  dioxide 
of  Venus.  Entry  points  are  on  Figure  5-9. 

Table  5-2  summarizes  the  sequence  of  some 
important  events  that  occurred  during  the 
entry  of  the  Pioneer  Venus  probes.  On  entry 
(Figure  5-10),  the  Large  Probe  decelerated  from 
41,800  to  727  km/hr  (25,975  to  452  mph) 
within  38  seconds.  During  this  period,  its 
onboard  memory  stored  data  for  later  trans- 
mission after  radio  blackout.  Its  parachute 
opened  at  10:45  a.m.  PST  to  further  slow  its 
speed  of  descent.  Its  forward  aeroshell  heat 
shield  jettisoned  to  expose  all  apertures  and 
windows  for  the  operation's  descent  phase. 
Forty-three  seconds  after  entry,  instruments  on 
the  Large  Probe  operated  normally  and 
returned  data  to  Earth.  This  was  at  an  altitude 

Table  5-2.   Important  Entry  Events 

Time  at  spacecraft3,  hr:min:sec,  PST 






End  of  coast  timing 
Initiate  telemetry 
200-km  (124-mile)  entry 
Radio  blackout  began 
Signal  locked  on 
Jettison  parachute 
Impact  with  surface 
Signal  ended 

Bus  entry  (200  km;  124  miles) 
Bus  signal  ended  (1  1  0  km;  68  miles) 








Descent  time  (entry  to  impact) 
Blackout  time  (signal  loss  to  relock) 
Time  on  parachute  (large  probe  only) 
Surface  operations  (impact  -  signal  end) 








aEarth-received  times  were  approximately  3  min  later  than  the  above  spacecraft  times. 

of  about  66  km  (41  miles).  Seventeen  minutes 
later,  at  11:02  a.m.  PST,  and  at  an  altitude  of 
45  km  (28  miles)  above  Venus'  hot  surface,  the 
probe  jettisoned  its  parachute  (see  Figure  5-10). 
Rotating  slowly  under  the  influence  of  its  spin 
vanes,  the  probe  continued  to  plunge  down. 
The  dense  atmosphere  slowed  its  descent,  just 
as  a  huge  metal  ball  would  slow  if  sinking  into 
Earth's  ocean. 

The  aerodynamically  stable  pressure  vessel 
reached  Venus'  surface  about  39  minutes  after 
the  probe  had  jettisoned  its  parachute.  The 
probe  hit  the  surface  at  only  32  km/hr  (20  mph). 
It  landed  near  Venus'  equator  on  the  dayside 
at  11:41  a.m.  PST,  some  55  minutes  after  first 
encountering  the  Venusian  atmosphere.  Its 
radio  signals  ended  abruptly  at  impact. 

Five  minutes  before  the  Small  Probes  encoun- 
tered the  peak  deceleration  pulse  of  atmo- 
spheric entry,  each  probe's  command  unit 
ordered  the  blackout  format.  This  stored 
spacecraft  data  in  an  internal  memory.  It  also 

stored  heat-shield  temperature  and  accelerom- 
eter  measurements  for  the  atmospheric  struc- 
ture experiment.  This  procedure  ensured  that 
no  data  were  lost  during  the  10-  to  15-second 
communications  blackout  at  entry.  The 
probes  transmitted  these  data  later  during 
the  descent. 

The  Small  Probes,  entering  the  atmosphere 
within  a  few  minutes  of  each  other,  quickly 
slowed  down.  This  occurred  between  10:50 
and  10:56  a.m.  PST.  The  atmosphere  retarded 
their  fall  to  the  surface  without  the  use  of 
parachutes.  Because  the  flightpath  angles  of 
the  three  Small  Probes  varied  considerably, 
each  probe's  deceleration  rate  and  entry 
heating  also  varied  widely.  Peak  decelerations 
ranged  from  220  to  456  g  (1  g  is  32  ft/sec/sec). 

At  10:51  a.m.  PST,  the  nephelometer's  window 
opened  on  the  North  Probe.  The  instrument 
began  to  gather  data  on  locations  and  densi- 
ties of  cloud  layers.  The  atmospheric  structure 
and  net  flux  radiometer  housing  doors  opened 


Figure  5- 1 0.  Entry  sequence  of 
the  Large  Probe  was  more 
complicated  than  that  of  the 
Small  Probes.  A  parachute  had 
to  slow  its  descent.  When  the 
parachute's  work  was  done,  the 
spacecraft  jettisoned  it. 


pilot  chute 

and  release  Aft 


chute  bag 


Deploy  main 




next.  These  instruments  started  telemetering 
to  Earth  data  about  the  atmosphere's  thermal 
structure.  Instrument  booms  deployed.  Within 
the  next  6  minutes,  similar  sequences  had 
started  on  the  other  Small  Probes. 

As  instrument  compartment  doors  opened  on 
either  side  of  each  Small  Probe's  afterbody, 
their  drag  effects  on  the  atmosphere  further 
reduced  each  spacecraft's  spin  rate.  A  small 
vane  on  the  pressure  inlet  prevented  the 
despin  rate  from  falling  to  zero.  This  would 
have  prevented  instruments  from  making 
observations  over  a  full  rotation  of  the  probe. 
Now  the  upper  descent  phase  began,  with  the 
three  probes  in  the  altitude  range  of  72  to  65  km 
(44  to  40  miles)  and  all  instruments  operating. 

As  the  probes  penetrated  deeper  into  thicker 
atmosphere,  it  interfered  with  radio  communi- 

cation. Signals  received  at  Earth  were  weakened. 
At  entry  plus  16.4  minutes  and  at  an  altitude 
of  about  30  km  (18  miles),  the  bit  rate  of  data 
transmission  from  probes  to  Earth  automati- 
cally reduced  to  16  bits/sec.  This  ensured  that 
Earth  stations  would  receive  data  from  the 
lower  atmospheric  regions.  The  DSN  now  had 
to  achieve  a  third  lockup  on  each  probe's 
transmission.  Again,  it  was  highly  successful, 
and  no  data  were  lost  in  the  process. 

From  that  point  on,  the  three  probes 
descended  into  Venus'  increasingly  dense 
atmosphere.  They  impacted  the  surface  at 
36  km/hr  (22  mph)  57  minutes  after  their 
entries.  Unlike  the  Large  Probe,  the  Small 
Probes  retained  their  heat  shields  to  the  sur- 
face. The  atmosphere's  density  is  so  great  that 
the  drag  of  these  aerodynamic  surfaces  slowed 
the  probes  to  their  desired  descent  speed. 

The  North  Probe  landed  at  11:47  a.m.  PST  in 
darkness  near  northern  polar  regions.  The 
Day  Probe  went  into  the  southern  hemisphere 
on  the  dayside  and  landed  at  11:50  a.m.  It 
kicked  up  a  dust  cloud  that  took  several 
minutes  to  settle.  The  Night  Probe  went  down 
in  darkness  onto  the  surface  in  the  southern 
hemisphere  at  11:53  a.m.  PST.  Signals  from 
the  North  Probe  and  the  Night  Probe  ended 
at  impact.  However,  transmissions  continued 
from  the  Day  Probe  for  another  68  minutes 
(Figure  5-11)  before  it,  too,  became  silent. 
Engineering  data  radioed  back  from  the  Day 
Probe  showed  that  its  internal  temperature 
climbed  steadily  to  a  high  of  126°C  (260°F). 
Then  its  batteries  were  depleted,  and  its  radio 
became  silent.  The  internal  pressure  monitors 
showed  that  the  pressure  within  the  probe  rose 
as  expected  for  a  sealed  bottle  on  the  surface 
of  Venus.  The  temperature  increase  gradually 
caused  an  expected  increase  in  internal  pres- 
sure. There  was  no  evidence  of  any  leaks  into 
the  probe  from  the  atmosphere  following  the 
impact.  It  was  clear  that  the  seals  had  with- 
stood the  real-life  test  of  impact  with  the  hot 
surface  of  Venus. 

Table  5-3  shows  the  locations  on  Venus  where 
the  probes  impacted  and  the  conditions  at  the 
impact  points.  These  locations  were  very  close 
to  the  points  targeted  before  the  probes  sepa- 
rated from  the  Bus. 

Meanwhile,  the  Multiprobe  Bus  hurtled  toward 
Venus  close  behind  the  probes.  On  December  8, 
controllers  reoriented  the  Bus  to  its  final  entry 
angle.  They  calibrated  its  instruments  and 
released  the  cap  covering  the  inlet  to  the 
neutral  mass  spectrometer.  Entry  was  sched- 
uled for  12:21  p.m.  PST  on  December  9.  This 
was  about  96  minutes  after  the  first  probe 
entered  and  88  minutes  after  the  last  probe 
had  entered. 

The  Bus  plunged  into  the  atmosphere  on  the 
planet's  dayside  at  a  high  latitude  in  the  south- 
ern hemisphere.  Table  5-4  gives  the  Bus'  entry 
position  at  an  altitude  of  200  km  (124  miles) 
and  the  locations  of  the  subsolar  and  sub- 
Earth  points.  These  are  the  points  on  Venus' 
surface  where  the  Sun  and  Earth  would  appear 
directly  overhead  to  an  observer. 

Since  the  Bus  had  no  heat  shield  to  protect  it 
from  high-speed  entry,  scientists  expected  to 
gather  data  for  only  2  minutes  before  it  burned 
up.  Radio  transmissions  from  the  Bus  poured 
back  to  Earth  carrying  scientific  data  at  a  rate 
of  1024  bits/sec.  These  data  carried  informa- 
tion about  the  composition  of  Venus'  very 
high  atmosphere,  including  the  region  where 
the  ionosphere  is  most  dense.  The  other 
probes  could  not  explore  this  region.  They 
could  gather  no  data  from  external  sensors 
until  they  had  been  slowed  by  the  atmosphere 
and  were  much  deeper  within  it. 

The  Bus  burned  up  at  12:23  p.m.  PST,  and  the 
uniquely  exciting  phase  of  the  entry  part  of 
the  mission  concluded.  It  had  lasted  for  only 

Figure  5-11.  This  is  how  a 
Pioneer  Venus  probe  might  have 
looked  on  the  hot  surface  of 
Venus.  Although  engineers  did 
not  design  the  probes  to 
withstand  impact,  there  was  a 
chance  that  one  might  survive 
and  transmit  some  data  back  to 
Earth.  One  Small  Probe  did 
survive  and  sent  data  from  the 
surface  for  67  minutes. 


Table  5-3.  Pioneer  Venus  Multiprobe  Impacts 




Longitude,  E 

Solar  zenith  angle, 

Local  Venus  time, 


4.4  N 
59.3  N 





31.  3  S 
28.7  S 




Table  5-4.  Pioneer  Venus  Bus  Entry  and  Location  of  Sun  and  Earth  Subpoints 



Longitude,  E 

Solar  zenith  angle, 

Local  Venus  time, 

Bus  entry  at  200  km 

37.9  S 
0.5  S 




1  2:00 

90  minutes.  Yet  in  that  short  period,  the  probes 
and  the  Bus  had  recorded  data  for  a  completely 
new  look  at  the  complex  atmosphere  of  Earth's 
sister  planet  (Figure  5-12).  During  the  following 
few  days,  scientists  completed  preliminary  data 
analysis  and  announced  some  unexpected 
discoveries.  Then  the  mission  settled  down  to 
the  equally  fascinating  but  more  lengthy  pro- 
cess of  observing  Venus  from  the  Orbiter.  This 
lasted  many  Venus  sidereal  days. 

There  were  major  findings  from  the  probes.  The 
four  probes  measured  the  atmosphere's  struc- 
ture, temperature,  pressure,  density,  and  wave 
structures.  They  started  at  altitudes  of  138  km 
(Large  Probe)  down  to  Venus'  surface.  The 
Small  Probes  measured  structure  from  133  km 
(Night  Probe),  126  km  (Day  Probe),  and  120  km 
(North  Probe).  Chapter  6  discusses  the  science 
results  in  detail.  However,  there  were  some 
discoveries  that  produced  much  excitement  in 
the  days  immediately  following  the  encounter. 

An  unexpected  result  was  concentrations  of 
primordial  argon  and  neon  several  hundred 
times  those  on  Earth.  This  finding  conflicted 
with  most  accepted  theories  about  the  origin 
of  the  Solar  System.  Those  theories  argued  that 
the  Sun  and  planets  formed  about  the  same 
time.  They  claimed  the  planets  gradually  grew 
through  planetesimals  and  planetary  embryos 
from  a  gas  cloud  surrounding  the  Sun  and 
composed  of  the  same  elements  as  the  Sun. 
The  next  chapter  discusses  the  isotopic 
findings  in  detail. 

Some  Puzzling  Results 

How  did  the  probes  and  their  instruments 
withstand  the  rigors  of  the  descent  into  Venus' 
atmosphere?  Scientists  had  been  concerned 
that,  when  the  probes  went  through  the 
clouds,  droplets  might  condense  on  the  inlet 
to  the  mass  spectrometer.  To  prevent  such 
contamination,  engineers  had  placed  a  heater 
coil  around  the  inlet.  Nevertheless,  the  inlet 
did  become  blocked,  and  observers  noticed  a 
change  in  the  amount  of  gas  entering  the 
instrument.  Later  in  the  descent,  when  the 
temperature  had  risen,  they  observed  peaks  of 
sulfur  in  the  data.  It  appeared  that  a  large  drop 
of  sulfuric  acid  had  blocked  the  inlet.  When  it 
later  boiled  off,  its  components  entered  the 
instruments  and  were  revealed  in  the  data. 

There  were  some  anomalies,  or  irregularities, 
with  all  the  probes.  Anomalous  events 
appeared  in  the  engineering  data  and  in  the 
science  data  at  approximately  the  same  alti- 
tude in  all  four  probes. 

The  first  signs  came  from  the  sensors  of  the 
atmospheric  structure  experiment  at  an  alti- 
tude between  12  and  14  km  (7.5  and  8.7 
miles).  Soon  afterward,  external  sensors  of  the 
net  flux  radiometer  on  the  North  Probe,  Day 
Probe,  and  Night  Probe  suddenly  failed  at 
approximately  the  same  altitude.  In  the  data 
from  other  scientific  instruments  and  from 
engineering  transducers,  other  anomalies 
occurred  just  before,  during,  and  after  these 
failures.  Table  5-5  summarizes  these  anomalies. 







•  Ice  crystal  haze  (mode  1) 

Suffuric  Acid 
Upper  cloud  c|oud  deck 

(modes  1,  2  &  3) 

Ionosphere  out  to  1000  km 

/-  Cloud  tops  - 1°  to  2° 

cooler  on  night  side 

Thin  smog 
(modes  1  &  2) 


360  kph  winds 
in  this  zone 




Middle  cloud  deck 

Lower  cloud  deck 

10  to  50  km  - 
atmosphere  convectively 
stable,  global  circulation 
patterns  horizontal  with 
rising  currents  at  Equator, 
descending  at  poles 

Slow  convection 


+  70 
+  90 



"^ — Thin  cloud  layers 

-  Thick  opaque  clouds,  sulfuric  acid     (modes  1  &  ^ 
(modes  1, 2  &  3) 

Haze  layer -Thin  likely 

sulfuric  acid  particles 


Atmosphere  is  clear 
below  30  km 


+  210 

+  380 

+  410 

Figure  5-1 2.  The  Pioneer  Venus 
mission  provided  a  detailed  and 
accurate  picture  of  Venus' 
atmosphere,  the  thick  cloud 
layers,  and  the  wind  systems. 

It  seems  unreasonable  to  assume  that  all  these 
different  instruments  failed  together  and  at 
precisely  the  same  condition.  A  cause  other 
than  simple,  virtually  simultaneous  equipment 
failure  seemed  likely. 

The  temperature  sensors  (Figure  5-13)  of  the 
atmospheric  structure  experiment  were  exposed 
to  Venus'  atmosphere,  and  they  showed 
anomalies.  However,  it  seemed  clear  from  the 
data  that  the  temperature  sensors  did  not 
physically  break  because  an  expected  electrical 
resistance  through  the  sensor  of  25  ohms 
remained.  Partial  shorting  of  the  insulation  of 
the  Tl  fine-wire  sensors  while  in  the  clouds 
indicated  continuous  acid  films  on  the  sensors. 
However,  this  cleared  as  the  probes  descended 
lower  into  higher  temperatures.  Also,  the 
shorting  effects  within  the  clouds  varied  for 
the  different  probes,  but  the  anomalies  all 
occurred  later  at  the  same  altitude.  That  is, 

they  occurred  at  the  same  temperature  and 
pressure  levels  in  the  atmosphere.  Moreover, 
the  Tl  and  T2  sensor  elements  exhibited 
anomalies  almost  at  the  same  time,  despite 
their  different  physical  configurations.  The  Tl 
sensors  each  consisted  of  a  coil  of  fine  plati- 
num wire  wound  on  a  frame.  The  T2  sensors 
were  more  robust.  They  consisted  of  platinum 
wire  bonded  as  a  resistance  thermometer  on 
top  of  a  thin  glass  insulating  layer.  It  is 
important  to  note  that  the  sensors  that  failed 
at  almost  the  same  time  were  made  of  different 
materials  and  that  their  electronics  were 
isolated  from  each  other. 

Another  anomaly  involved  the  sensor  boom 
for  the  atmospheric  structure  and  net  flux 
radiometer  experiments.  Its  telemetered 
change  from  deployed  to  stowed  position  was 
a  mechanical  impossibility.  Investigators  made 
a  post-flight  analysis  of  identical  boom  status 


Table  5-5.  Anomalies  Experienced  by  Probes 






Apparent  failure  of  temperature  sensors 





Apparent  failure  of  net  flux  radiometer  fluxplate 




temperature  sensors 

Abrupt  changes  and  spikes  in  data  from  net  flux  radiometer 




Change  in  the  indicated  deployment  status  of  the  atmosphere 




structure  temperature  sensor  and  net  flux  radiometer  booms 

Erratic  data  from  two  thermocouples  embedded  in  the 




heat  shield 

Erratic  data  from  a  thermistor  measuring  junction 




temperature  of  the  heat-shield  thermocouples 

Slight  variation  of  current  and  voltage  levels  in  the  power  bus 




Abrupt  changes  in  cloud  particle  size  laser  alignment  monitor 





Decrease  in  the  intensity  of  the  beam  returned  to  the 





cloud-particle-size  spectrometer 

Noise  in  the  data  from  the  infrared  radiometer 





Spikes  in  the  data  monitoring  the  ion  pump  current  of  the 





mass  spectrometer  analyzer 

Spurious  reading  from  the  thermocouples  when  the  heat  shield 





was  dropped  from  the  probe 


switches.  They  concluded  that  failure  of  these 
switches  under  conditions  of  high  temperature 
and  pressure  was  a  likely  cause. 

Investigators  also  had  an  explanation  for 
anomalies  in  the  Large  Probe's  housekeeping 
data,  particularly  the  strange  readings  from  the 
heat-shield  thermocouple  and  thermistor.  They 
reasoned  the  probe  became  covered  with  a 
plasma  of  charged  particles,  but  scientists  no 
longer  consider  this  likely.  An  apparent  reading 
from  a  thermocouple  in  the  Large  Probe's  heat 
shield  occurred  after  the  leads  had  severed  and 
the  heat  shield  became  detached.  Somehow  an 
electrical  potential  of  0.2  mV  had  been  created 
between  the  ends  of  the  severed  leads.  This 
potential  exhibited  slight  changes  during  the 
rest  of  the  descent  to  the  surface.  One  sugges- 
tion was  that  the  severed  leads  acted  as  a 
Langmuir  probe  in  a  plasma.  However,  there 
was  no  known  source  for  such  a  plasma. 

If  they  could  have  occurred,  static  discharges 
within  or  outside  the  probe  might  explain 
several  anomalies.  These  included  anomalies 

of  changes  in  the  Large  Probe's  transponder 
static  phase  error  and  receiver  automatic  gain 
control.  They  also  could  explain  jumps  in 
internal  pressure  and  temperature  readings. 

Investigators  considered  charge  buildup  on  the 
probes,  but  the  nephelometer  showed  a  clear 
atmosphere  below  40  km  (25  miles).  So  a  major 
question  was  how  such  a  charge  might  build 
up  in  a  particle-free  atmosphere.  Although  the 
atmosphere  was  optically  clear,  it  might  be 
ionized.  It  could  literally  be  swarming  with 
submicroscopic  ions  created  by  cosmic-ray  reac- 
tions at  the  molecular  levels  as  opposed  to  the 
particle  level.  Such  chemical  reactions  could 
build  up  a  charge  in  a  clear  atmosphere.  How- 
ever, the  existence  of  these  anomalies  is  now 
in  question.  In-depth  review  suggests  the  mea- 
surements were  within  normal  operating  limits. 

The  diamond  window  heater  for  the  infrared 
flux  radiometer  burned  out.  The  instrument 
measured  data  from  the  window  frame,  and 
this  gave  a  spurious  input.  Investigators  first 
thought  it  was  an  anomaly. 

One  possible  cause  for  the  window  heater  fail- 
ure is  related  to  the  tantalum  heater  sheath.  At 
high  temperatures,  there  is  a  reaction  between 
tantalum,  carbon  dioxide,  and  acid.  Both  of 
the  latter  are  present  in  quantity  in  the 
Venusian  atmosphere.  Engineers  speculate  that 
holes  developed  in  the  tantalum  heater  sheath 
from  such  a  reaction.  The  insulation  could  have 
then  become  contaminated  enough  to  provide 
conductive  paths.  Such  paths  could  have 
allowed  an  electrical  short  between  the  heater 
and  the  spacecraft  ground.  This  would  have 
shorted  the  heater  circuit  and  blown  its  fuse. 

Several  factors  can  explain  most  of  the  probe 
anomalies.  These  include  effects  arising  from 

an  unexpected  electrical  interaction  between 
the  probes  and  the  atmosphere,  or  from 
chemical  reactions  between  the  atmospheric 
gases  and  probe  materials.  The  source  for  a 
reaction  of  such  widespread  effect  is,  however, 
still  uncertain. 

The  performance  of  these  probes  in  the 
extremely  inhospitable  atmosphere  of  Venus 
was  remarkable.  They  gathered  a  wealth  of 
important  new  data  just  as  project  scientists 
planned.  Also,  technology  had  been  proved  for 
penetrating  planetary  atmospheres  and 
gathering  data  under  conditions  of  extremely 
high  temperatures  and  pressures.  This  new 
technology  held  the  potential  for  exploring 

25  urn  wire  sensor 
bonded  to  front  of 
Pt  tubing 

Free-wire  sensor, 
0.1  mm  Pt  wire 

Frame,  Pt  Rh 


Thin  walled, 
stainless  steel 
support  post 

3-axis  accelerometer 




Stagnation  pressure  inlet 


Stimulus  and 
sense  leads 




pressure  inlet 

Small  probes 

Figure  5-1 3.  As  the  probes 
reached  deep  into  the 
atmosphere,  several  instruments 
produced  unexpected  readings. 
These  included  the  atmospheric 
structure  temperature  sensors  in 
this  figure.  Sensors  of  entirely 
different  design  produced  bizarre 
results  at  the  same  altitude. 



the  many  bizarre  atmospheres  of  the  planets  in 
the  outer  Solar  System. 

Further  Analysis  of  the  Anomalies 

A  workshop  meeting  held  at  NASA  Ames 
Research  Center  on  September  28  and  29,  1993, 
reviewed  these  probe  anomalies  again.  This 
was  done  in  connection  with  planning  for  the 
design  of  a  Discovery  Venus  Probe.  Participants 
included  probe  system  engineers,  project  office 
personnel  (retired  and  active),  probe  scientists, 
instrument  designers,  and  atmospheric  scien- 
tists. These  latter  included  chemists,  dynam- 
icists,  and  electrodynamicists. 

Workshop  attendees  reviewed  anomalies  that 
occurred  at  or  below  12.5  km  (7.75  miles)  in 
detail.  As  this  chapter  described  earlier,  instru- 
ments outside  the  sealed  pressure  vessels  had 
exhibited  problems  on  all  four  probes.  Tem- 
perature sensors  had  continued  to  report  data, 
but  not  valid  data.  Net  flux  radiometers  had 
shown  a  sudden  decrease  in  net  flux  toward 
zero.  Also,  box  cover  status  signals,  on  boxes 
from  which  the  temperature  and  net  flux 
radiometer  had  been  deployed  on  the  Small 
Probes,  had  indicated  the  sensors  had  been 
restored.  This  was  an  impossibility.  The  Large 
Probe's  thermocouple  wire,  cut  before  para- 
chute deployment,  had  indicated  signals  of  a 
few  millivolts.  By  contrast,  the  scientific  data 
from  all  internal  sensors  continued  without 
anomaly  throughout  the  descent. 

The  workshop  clarified  the  anomalies.  It  also 
corrected  some  mistaken  impressions  circu- 
lated mainly  by  word  of  mouth  earlier  during 
the  mission.  Participants  credibly  accounted 
for  a  few  of  the  anomalies  during  Phase  I.  Yet, 
despite  many  speculations  and  suggestions, 
there  was  no  clear-cut  explanation  for  the 
remaining  array  of  nearly  simultaneous  events. 

The  workshop  participants  considered  several 
atmospheric  phenomena  to  explain  these 
anomalies.  All  appeared  possible  but  needed 
further  investigation.  They  were: 

1)  Chemical  interactions  such  as  clouds  acting 
on  the  harness  and  sensors  to  produce  sulfuric 
acid  and  carbon  dioxide  oxidation  of  titanium 
parts  and  harness  materials. 

2)  Conductive  vapors  condensing  on  the 
external  sensors  in  the  deep  atmosphere, 
leading  to  electrical  shorts. 

3)  Probe  charging  with  subsequent  electrical 
breakdown  of  the  atmosphere,  possibly  leading 
to  sparks  that  could  ignite  fires  in  external 
materials  such  as  the  Kapton  insulation.  Also, 
many  metals  burn  in  carbon  dioxide,  and  the 
flammability  increases  with  increasing  pres- 
sure. For  example,  zirconium,  magnesium,  and 
titanium  ignite  easily  in  pure  carbon  dioxide. 

Soviet  Venera  and  Vega  probes  and  landers 
also  carried  many  external  instruments.  Tita- 
nium was  an  important  element  in  their  con- 
struction, too.  Soviet  scientists  have  stated, 
however,  these  spacecraft  did  not  experience 
anomalous  behavior.  It  seems  that  particular 
probe  or  instrument  design  features  must 
explain  the  Pioneer  Venus  anomalies.  Investi- 
gators needed  to  identify  these  features. 

The  workshop  concluded  that  although  the 
data  are  not  now  sufficient  for  conclusive 
proof,  investigators  have  identified  the  most 
probable  causes  of  the  anomalies.  The  most 
likely  hardware  event  is  insulation  breakdown 
of  the  external  harness.  This  resulted  from 
chemical  interaction  with  the  high  tempera- 
ture and  pressure  of  the  carbon-dioxide  atmo- 
sphere after  exposure  to  the  clouds  of  sulfuric 
acid.  Laboratory  testing  before  the  workshop 
had  not  ruled  out  that  possibility.  The  prob- 
able interaction  between  the  probe  and  the 

atmosphere  resulted  in  a  charge  buildup  dur- 
ing transit  through  the  clouds  (with  charge 
retention  to  breakdown  occurring  at  the 
anomaly  altitude). 

The  workshop  recommended  that  investiga- 
tors continue  to  try  to  reproduce  these  anoma- 
lous effects  and  attribute  their  cause  to  a  few 
credible  explanations.  This  testing  also  would 
have  the  potential  of  identifying  other  possible 
atmospheric  interactions.  Engineers  then 
could  design  deep  atmosphere  probes  that 
would  not  succumb  to  these  anomalies. 

Nominal  Mission  of  the  Orbiter 

Preliminary  science  discoveries  came  from  the 
Orbiter  experiments.  Data  from  the  Orbiter's 
first  radar  map  (Figure  5-14)  suggested  that 
Venus'  topography  might  be  similar  to  Earth's. 
The  data  revealed  high  features  similar  to 
mountains  and  extensive,  relatively  flat  areas. 
Some  of  the  radar  mapper's  first  preliminary 
scans  were  in  a  region  of  Venus  previously 
unexplored  by  radar.  This  was  a  strip  that 
extends  for  about  1900  km  (1180  miles).  In 
this  region,  much  of  the  surface  appeared 
relatively  flat.  It  was  similar  to  Earth's  surface 
and  quite  different  from  the  rough,  cratered 
surfaces  of  Mars,  Mercury,  and  the  Moon. 

After  the  first  two  dozen  orbits,  a  serious  set- 
back occurred.  The  radar  instrument  stopped 
working.  Teams  of  scientists  and  engineers 
tried  several  remedies,  but  to  no  avail.  This 
failure  greatly  disappointed  everyone  because 
the  radar  had  started  to  reveal  tantalizing 
details  of  the  planet's  surface.  When  all  correc- 
tive measures  failed,  controllers  turned  off  the 
radar  mapper.  During  the  down  time,  mission 
scientists  analyzed  the  instrument's  design. 

However,  they  came  up  with  no  additional 
corrective  ideas.  Yet,  when  controllers  turned 
on  the  radar  again  a  month  later,  it  worked 

(although  not  quite  normally).  The  problem 
seemed  transient,  associated  with  operating 
the  instrument  for  periods  longer  than  10  hours. 
Controllers  had  operated  the  instrument  for 
the  first  orbits  and  not  turned  it  off.  Analysis 
led  to  the  conclusion  that  an  electrical  charge 
may  have  accumulated  in  its  sensitive  logic 
circuitry.  So  the  experiment  team  leader, 
Gordon  Pettengill,  and  project  personnel 
decided  to  use  new  operating  modes  for  the 
instrument.  During  each  orbit,  they  operated  it 
for  a  while  and  then  turned  it  off.  This  peri- 
odic use  resulted  in  normal  operation  of  the 
radar  mapper  within  about  10  days.  Afterward, 
it  operated  satisfactorily.  Although  this  failure 
caused  a  month  of  radar  data  loss,  the  extended 
mission  later  covered  the  missed  areas. 

Another  disappointment  with  Pioneer  Orbiter 
was  not  as  happily  resolved.  The  infrared 

Figure  5- 1 4.  Pioneer  Orbiter's 
first  radar  scans  of  Venus' 
surface  produced  intriguing 
new  maps  of  the  cloud-hidden 
surface.  The  instrument  also 
measured  elevations  and 
revealed  enormous  moun- 
tains, continental  masses, 
and  deep  valleys. 


Table  5-6.  Orbital  Parameters  for  Nominal  Mission 



Periapsis,  km  (miles) 

Apoapsis,  km  (miles) 


Average  period,  hr 

Inclination  to  equator,  deg 

Periapsis  latitude,  deg 

Periapsis  longitude,  deg  (for  orbit  5) 




1  70.2  E 

radiometer  failed  when  the  spacecraft  was 
on  about  its  seventieth  orbit.  Despite  many 
attempts  to  correct  the  failure,  personnel  could 
not  bring  the  instrument  back  into  operation. 
Investigators  believed  that  the  problem  arose 
in  the  instrument's  power  supply. 

Other  instruments  experienced  minor  prob- 
lems from  time  to  time,  but  all  were  resolved. 
The  instruments  recovered  quickly,  and  they 
gathered  data  throughout  the  mission. 

Mission  planners  selected  the  initial  altitude 
of  periapsis  high  enough  for  negligible  atmo- 
spheric drag  on  the  spacecraft  during  the  first 
orbit.  They  had  to  choose  a  very  conservative 
altitude  because  information  about  Venus' 
upper  atmosphere  was  sparse.  As  they  received 
information  from  the  spacecraft,  controllers 
commanded  seven  periapsis  correction  maneu- 
vers during  the  first  10  orbits.  These  reduced 
the  periapsis  to  the  scientifically  desired  150  km 
(93  miles)  above  the  mean  surface  of  Venus.  In 
this  way,  they  achieved  the  orbital  parameters 
for  the  nominal  mission  (Table  5-6). 

Perturbations  from  the  Sun's  gravity  field 
affected  the  periapsis  position  of  the  orbit. 
This  required  control  by  thrusters  to  maintain 
the  variations  in  altitude  within  predeter- 
mined limits.  Without  corrections  to  the  orbit 
by  use  of  these  thrusters,  the  Sun's  gravity 
would  have  pushed  the  periapsis  out  from 
the  planet.  That  is,  it  would  have  raised  its 
altitude.  To  keep  the  periapsis  within  the  range 

of  altitudes  desired  by  the  scientists,  periodic 
corrections  were  required  throughout  the 
entire  nominal  mission. 

Figure  5-15  shows  a  plot  of  periapsis  altitude 
for  the  early  part  of  the  mission.  It  illustrates 
how  the  altitude  of  periapsis  changed  through 
the  nominal  and  into  the  extended  mission. 
During  the  first  few  weeks  of  the  spacecraft's 
operation  in  orbit,  controllers'  commands  to 
lower  the  periapsis  to  150  km  (93  miles)  were 
issued  before  it  passed  from  the  dayside 
to  the  nightside  of  Venus.  The  atmosphere  is 
less  dense  on  the  nightside  of  the  planet  than 
on  its  dayside.  Because  of  this,  they  com- 
manded the  periapsis  several  times  to  142  km 
(88  miles)  while  it  was  on  the  nightside  to 
allow  the  spacecraft  to  sample  deeper  into 
the  atmosphere. 

The  Orbiter  was  oriented  with  its  spin  axis  per- 
pendicular to  the  ecliptic  plane.  The  despun 
antenna  was  to  the  south  end  of  the  space- 
craft. This  orientation  continued  through  the 
mission,  except  for  several  short  periods  to 
observe  comets.  Initially,  the  view  of  the  north 
polar  region  was  better  than  that  of  the  south 
polar  region.  Two  factors  accounted  for  this. 
First,  the  scientific  instruments  were  on  an  equip- 
ment shelf  near  the  antenna's  base  and,  second, 
periapsis  occurred  at  a  northern  latitude. 

Figure  5-16  shows  how  some  orbit  relation- 
ships varied  during  the  nominal  243-day 
mission.  The  Sun-Venus-Pioneer  orbit  system 



E  180 





2  170 






"o      ££ 

o.     c  c 
'—"     <u  <u 

•5        Mr- 


i  i  r 

Periapsis  eclipse 




9- u 

<    01 


13  a 



Venus  local  time  of  periapsis 





















i  i 

20  40  60  80  100  120  140 

Periapsis  number 

I    I  I  _  I  III  _  |     |     | 






49        22  28  1 

Dec  Jan 

16  23   1 




7  13 


7       17 


1  3  910 









1  4 

appears  at  four  positions  in  the  sidereal  year 
from  December  9,  1978,  to  July  22,  1979. 
Since  the  orbit  was  fixed  in  an  inertial  refer- 
ence frame,  the  lines  of  apsides  remained 
"parallel"  to  one  another  at  each  of  these  four 
positions.  The  local  time  of  periapsis  increased 
by  1.6°  each  Earth  day.  At  periapsis,  the 
Orbiter  first  sampled  the  dayside  upper  atmo- 
sphere of  Venus.  Then,  after  several  weeks  of 
moving  at  1.6°  per  day,  the  periapsis  crossed 
the  evening  terminator.  Now  the  spacecraft 
sampled  the  nightside  atmosphere  and 
ionosphere  at  each  periapsis.  Later  still,  the 
periapsis  crossed  the  morning  terminator,  and 
the  spacecraft  sampled  the  dayside  again.  The 
spacecraft  crossed  the  evening  terminator 

again  at  the  end  of  the  nominal  mission. 
Instruments  thus  obtained  data  at  periapsis 
and  along  the  orbit  for  all  Venus  local  times  in 
a  period  of  224.7  Earth  days. 

However,  because  of  Venus'  retrograde  axial 
rotation,  the  longitude  of  periapsis  moved  rela- 
tive to  the  solid  body  of  the  planet  at  1.48°  per 
day,  that  is,  per  orbit.  So,  the  spacecraft  needed 
243  Earth  days  to  observe  all  longitudes  on  the 
solid  planet.  The  Orbiter  completed  its  nomi- 
nal mission  on  August  4,  1979.  It  had  con- 
served enough  propellant  to  stay  in  orbit  for  at 
least  another  two  sidereal  periods.  That 
amounted  to  another  486  days.  In  fact,  it 
operated  for  many  sidereal  days.  This  provided 

Figure  5-75.  This  plot  shows  the 
altitude  of  the  periapsis  during 
the  nominal  mission  of  the 
Orbiter  and  partway  into  the 
extended  mission.  The  periods  of 
eclipses  and  occultations  are        -.  r-i 

Figure  5- 1 6.  This  drawing  of  the 
Sun-Venus-Orbiter  geometry 
illustrates  how  the  periapsis 
moved  around  the  planet  during 
the  Venusian  sidereal  year  to 
sample  day  and  night  hemi- 
spheres. Because  the  planet 
rotates  in  a  retrograde  direction, 
more  than  one  Venusian  sidereal 
year  was  required  for  periapsis  to 
move  over  all  longitudes  of  the 

Mar  31,  1979 

Mayl,  1979 

Marl,  1979 

May  27,  1979 

Feb  3,1979 

p         .^         Jan  1,1979 
224. 7d 


Dec  9,  1978 

(July  22,  1979) 

Dec  9,  1978 



a  tremendous  scientific  bonus  from  a  relatively 
inexpensive  planetary  mission. 

The  Science  Steering  Group  (SSG)  and  mission 
management  decided  to  continue  the  basic 
periodic  control  of  the  orbit  until  about  orbit 
600  on  July  27,  1980.  Then  they  allowed  the 
periapsis  altitude  to  rise  slowly.  Initially  it  rose 
at  a  rate  of  400  km  (249  miles)  each  243  days. 
By  1984,  it  was  rising  at  only  225  km 
(140  miles)  each  243  days.  The  apoapsis 
descended  at  an  identical  rate,  and  the  period 
of  the  orbit  remained  constant. 

The  Extended  Mission  ofOrbiter 

The  Pioneer  Venus  Orbiter  reached  Venus  on 
December  4,  1978,  and  controllers  placed  it 
into  a  highly  eccentric  orbit.  Then  they  used 
changes  in  the  altitude  of  periapsis  as  the  basis 
for  dividing  the  mission  into  three  separate 
phases.  Phase  I  was  the  initial  19  months 
when  controllers  maintained  periapsis  at  low 
altitudes  of  about  150  km  (93  miles).  Phase  II 
began  when  propellant  began  to  run  low.  Solar 
gravitational  perturbations  were  then  allowed 
to  cause  periapsis  to  rise  out  of  the  thermo- 
sphere  and  the  main  ionosphere.  Eventually, 

in  1986,  periapsis  reached  an  altitude  of  about 
2300  km  (1430  miles).  It  then  started  to 
descend.  Phase  III,  or  the  Entry  Phase,  began 
in  April  1991.  This  was  when  periapsis  was 
below  1000  km  (620  miles)  and  instruments 
made  direct  measurements  within  the  main 

Project  management  changed  for  Phase  II. 
Richard  O.  Fimmel  became  Pioneer  Project 
Manager  when  Charles  Hall  retired  from  NASA. 

A  solar  gravitational  effect  caused  periapsis  to 
rise  during  the  first  half  of  Phase  II  and  then  to 
fall  during  the  second  half  of  Phase  II  and  into 
Phase  III.  It  acted  in  a  distinct  cyclic  fashion. 
There  were  periods  of  decline  interrupted  twice 
each  Venus  year  by  increases  in  altitude.  These 
took  the  shape  of  S-curves  (Figure  5-17).  They 
occurred  as  the  orbit  plane  of  the  spacecraft 
passed  nearly  perpendicular  to  the  Sun.  Each 
cycle  was  associated  with  a  12-hour  sweep  of 
local  solar  time.  These  altitude  and  local  time 
changes  generated  opportunities  to  observe 
various  phenomena  of  scientific  importance 
occurring  in  Venus'  environment. 

By  the  middle  of  1992,  periapsis  was  again  low 
enough  for  instruments  to  resume  making 
measurements  within  the  ionosphere  and 
thermosphere.  They  also  were  able  to  scan  the 
limb  and  observe  the  thermosphere  in  ultra- 
violet light.  Scientists  had  an  equally  impor- 
tant Phase  III  goal.  They  wanted  to  extend  the 
observations  into  much  denser  atmospheric 
regions  than  was  acceptable  during  Phase  I. 
Also,  during  the  final  phase  of  Orbiter's 
mission,  instruments  made  measurements  at  a 
different  part  of  the  solar  cycle.  These  occurred 
within  those  higher  altitude  regions  that  the 
spacecraft  examined  earlier  in  the  mission. 

The  most  critical  part  of  Phase  III  was  the 
period  of  final  encounter,  which  began  early 

in  September,  1992.  During  that  month,  navi- 
gators used  most  of  the  remaining  hydrazine 
propellant  in  a  series  of  maneuvers  to  lift  peri- 
apsis. Mission  controllers  and  scientists  hoped 
that  sufficient  propellant  remained  to  delay 
entry  long  enough  to  reach  the  next  S-curve. 
Then  they  would  not  need  further  maneuvers 
to  maintain  the  altitude  of  periapsis.  At  that 
time,  periapsis  would  move  to  the  planet's 
dayside.  This  would  correspond  to  about 
100  orbits  (100  days)  in  the  range  of  orbit 
numbers  5020  to  5120.  If  additional  propellant 
remained  after  the  last  periapsis  maneuver,  mis- 
sion planners  intended  to  use  it  at  the  end  of 
the  S-curve.  That  would  probably  happen  in  the 
middle  of  December,  1992.  The  extra  propel- 
lant would  extend  measurements  further  into 
the  midday  thermosphere  and  ionosphere. 

In  1989,  NASA  established  a  task  force  to 
identify  the  most  important  scientific  goals  for 
the  Entry  Phase.  It  also  provided  the  Pioneer 
Project  Office  (PPO)  with  operational  guide- 
lines on  how  the  mission  might  best  use  the 
Orbiter  and  its  instruments  to  achieve  these 
goals.  The  guidelines  and  goals  had  to  stay 
within  the  limitations  of  the  orbit,  the  space- 
craft, and  the  DSN.  Detailed  planning  was 
important  to  take  full  advantage  of  the 
measurement  opportunities  at  that  time. 

The  local  time  and  altitude  of  periapsis  largely 
determined  the  kinds  of  phenomena  the 
spacecraft  could  encounter.  At  the  same  time, 
orbital  mechanics  placed  important  constraints 
on  the  quantity  and  quality  of  the  data.  For 
example,  the  telemetry  bit  rate,  which  con- 
trolled the  temporal  resolution  of  the  measure- 
ments, varied  widely  with  the  distance  from 
Earth  to  Venus.  Also,  instruments  could  not 
retrieve  data  near  solar  conjunction.  This  was 
because  DSN  antennas  picked  up  solar  radio 
noise  as  Venus  moved  close  to  the  Sun  (as 
viewed  from  Earth). 


Figure  5- 1 7.  The  three  stages  of 
the  Pioneer  Venus  Orbiter 
mission  appear  plotted  below  the 
graph  of  solar  activity  for  the 
same  period.  Phase  I  provided  in 
situ  measurements  deep  into  the 
thermosphere  and  the  main 
body  of  the  ionosphere  at  solar 
maximum.  The  changing 
altitude  of  the  periapsis  during 
Phase  II  permitted  the  spacecraft 
to  explore  the  upper  ionosphere, 
magnetosheath,  and  bow  shock. 
Phase  III  allowed  instruments  to 
reexamine  the  ionosphere  and 
thermosphere  at  moderate  levels 
of  solar  activity  and  down  to 
lower  altitudes  than  during 
Phase  I. 


0  L- 

,1979.1980,1981  ,1982,1983  ,1984,1985  ,1986  ,1987  ,1988  ,1989  ,1990  ,1991  ,1992, 

Calendar  year 

Orbit  injection 




Occultations  of  the  space  telemetry  signal  by 
Venus  also  limited  the  measurement  resolution 
near  periapsis.  Science  data  obtained  at  such 
times  remained  in  the  spacecraft's  Data  Storage 
Unit  (DSU).  Communications  equipment 
transmitted  the  information  to  Earth  later.  The 
DSU's  limited  capacity  required  a  compromise 
between  full  coverage  of  the  occultation  period 
(up  to  24  minutes)  and  higher  spatial  resolution 
during  only  part  of  the  occultation  period. 

Solar  eclipse  periods  reduced  the  solar  array's 
ability  to  recharge  the  spacecraft's  batteries.  As 
a  result,  those  periods  affected  the  time  that 
controllers  could  turn  on  instruments,  or  the 
number  of  instruments  that  could  be  used 
during  a  specific  orbit.  These  factors  affected 
the  planning  of  Orbiter's  operations  during 
Phase  III,  in  addition  to  the  scientific  opportu- 
nities provided  by  the  changing  local  time  and 
altitude  of  periapsis. 

Several  other  factors  made  the  task  of  space- 
craft operations  more  difficult  during  Phase  III. 
The  declining  solar  cell  capability  required  the 
electrical  energy  budget  to  be  more  carefully 
balanced  against  desired  scientific  goals.  Most 
science  goals  required  measurements  from 
many  instruments  at  the  same  time.  So,  time- 
sharing did  not  offer  significant  power  reduc- 
tions. Controllers  conserved  energy  within  the 
spacecraft  with  several  methods.  They  scheduled 
briefer  intervals  of  operation  about  periapsis, 
and  they  reduced  spacecraft  operations  at 
higher  altitudes  in  the  orbit. 

Another  complicating  factor  was  the  scientists' 
desire  to  obtain  as  many  measurements  as  pos- 
sible at  very  low  altitudes.  Maintaining  the 
orbit  for  this  purpose  required  that  spacecraft 
maneuvers  should  occur  every  few  days.  This 
was  in  contrast  to  the  weekly  maneuvers  navi- 
gators practiced  during  Phase  I. 

Mission  personnel  adopted  a  power-sharing 
plan  during  Phase  II.  The  plan  ensured  that 
controllers  turned  on  the  right  instruments  at 
the  right  places  in  the  orbit  and  in  the  correct 
local  time  sectors.  This  plan  changed  periodi- 
cally to  reflect  reductions  in  the  available  elec- 
trical power  and  changes  in  the  altitude  of 
periapsis.  An  Entry  Science  Plan  for  Phase  III 
served  the  same  purpose  as  the  Phase  II  power- 
sharing  plan.  However,  it  represented  a  more 
careful  attempt  to  focus  spacecraft  operations 
on  the  unique  scientific  goals  of  the  final  phase. 

In  September  1990,  a  group  of  eight  authors 
completed  a  report  on  the  plan,  entitled  The 
Pioneer  Venus  Orbiter  Entry  Science  Plan.  The 
authors  were  L.  H.  Brace,  University  of  Michi- 
gan (Chairman),  R.  W.  Jackson,  NASA  Ames 
Research  Center  (Co-Chairman),  G.  M.  Keating, 
NASA  Langley  Research  Center,  L.  E.  Lasher, 
NASA  Ames  Research  Center,  D.  W.  Lozier, 
NASA  Ames  Research  Center,  H.  B.  Niemann, 

NASA  Goddard  Space  Flight  Center,  A.  I.  F. 
Stewart,  University  of  Colorado,  and  R.  J. 
Strangeway,  University  of  California,  Los  Angeles. 

This  Entry  Science  Plan  reflected  the  consensus 
of  the  SSG  on  other  operational  matters  as  well. 
Among  them  was  how  the  project  should  use 
the  remaining  propellant  to  control  the  alti- 
tude of  periapsis.  Approaches  to  set  the  relative 
priority  among  investigations  that  may  have 
conflicting  operations  requirements  were  also 
included.  The  need  was  identified  for  an  Entry 
Encounter  Activity  at  the  end  of  1992,  with 
participation  by  appropriate  investigator  groups. 
The  Plan  also  identified  the  need  for  a  post- 
entry  data  analysis  period.  During  this  time, 
the  various  investigators  would  share  the  mea- 
surements from  Phase  III.  Also,  the  Plan  detailed 
the  data  format  for  submission  to  the  National 
Space  Science  Data  Center  for  archiving. 

The  project  established  several  operational 
guidelines  for  the  final  entry  phase.  The 
primary  scientific  interest  was  to  gather  data 
within  the  atmosphere  when  the  spacecraft 
was  near  periapsis.  The  relatively  brief  periapsis 
operations  did  not  require  much  orbit-averaged 
power.  Solar-cell  charging  current  and  battery 
capacity  were  devoted  to  operating  the  space- 
craft. This  included  powering  all  desired  instru- 
ments for  an  hour  or  so  near  every  periapsis 
passage.  When  the  DSN  was  not  available  to 
the  Orbiter,  the  DSU  on  the  spacecraft  stored 
the  periapsis  data  for  playback  later. 

However,  other  constraints  made  it  necessary 
to  settle  for  even  briefer  data  gathering  periods 
during  each  orbit.  Periapsis  passages  through 
Venus'  shadow  caused  deep  discharges  of  the 
battery.  This  worsened  if  all  applicable  instru- 
ments were  turned  on.  Care  in  husbanding  all 
resources  aboard  the  Orbiter  was  necessary. 
Scientists  and  mission  managers  wanted  the 
spacecraft  to  survive  the  first  periapsis  lifting 



interval  in  September-October  1992  (orbits 
5020  to  5070).  This  would  provide  a  chance  to 
obtain  dayside  measurements  in  the  December 
1982  to  January  1993  interval  (orbit  5100 
to  entry). 

A  question  arose  whether  measurements  were 
needed  on  every  orbit  periapsis  in  the  intervals 
that  the  scientific  goal  statements  called  for. 
Nearly  all  regions  of  the  Venusian  thermo- 
sphere  and  ionosphere  are  highly  dynamic  as 
they  respond  to  changing  solar  radiation  and 
solar-wind  conditions.  Each  periapsis  passage 
provided  a  snapshot  of  the  conditions  existing 
along  that  orbit  at  that  time.  Only  through 
comparisons  of  profiles  taken  under  diverse 
solar  conditions  could  researchers  hope  to  sort 
out  the  sources  of  the  observed  variations,  and 
periapsis  passages  occurred  only  at  24-hour 
intervals.  From  these  arguments,  managers 
concluded  that  they  should  avoid  the  unneces- 
sary omission  of  even  one  orbit,  if  possible.  The 
experimenters  decided  that  all  the  aeronomy 
instruments,  which  contributed  such  useful 
data,  should  operate  during  every  periapsis 
passage.  This  would  be  the  plan  unless  unfore- 
seen limitations  in  spacecraft  power  or  telem- 
etry made  this  operation  impossible  or  unwise. 

The  above  reasoning  had  an  important  corol- 
lary that  guided  the  selection  of  instrument 
operation  modes  for  each  particular  passage. 
Three  instruments  had  several  modes  of 
operation  for  acquiring  specific  kinds  of 
measurements  at  the  expense  of  others.  These 
instruments  were  the  retarding  potential 
analyzer,  ultraviolet  spectrometer,  and  neutral 
mass  spectrometer.  Because  several  investiga- 
tions shared  the  same  orbit  intervals,  experi- 
menters had  to  compromise.  Instrument 
investigators  were  made  responsible  for 
coordinating  their  plans  to  select  instrument 
modes  for  each  orbit.  Each  investigator  had  to 
justify  use  of  any  special  instrument  modes. 

Often,  factors  restricting  the  science  goals  were 
limitations  in  the  available  electrical  power, 
the  telemetry  rate  at  that  time  in  the  mission, 
and  the  occurrence  of  occultations  that  limited 
the  quantity  of  data  within  the  DSU.  Limita- 
tions in  the  DSU's  capacity  forced  a  choice 
between  receipt  of  a  low  data  rate  and  an 
intermediate  or  high  data  rate  in  a  slow  burst. 

Some  general  guidelines  came  from  a  variety 
of  sources.  They  emerged  from  scientific  goal 
statements  or  evolved  from  discussions  within 
the  Entry  Operations  Task  Force.  Others  were 
generalized  from  many  discussions  at  earlier 
SSG  meetings.  Periapsis  science  had  the 
highest  priority  during  Phase  III.  As  a  result, 
brief  uses  of  instruments  to  obtain  in  situ  and 
remote  measurements  of  the  atmosphere  and 
ionosphere  had  higher  priority  than  apoapsis 
scientific  goals.  The  mission  accepted  electron 
temperature  probe  and  plasma  analyzer 
measurements  an  hour  or  two  before  periapsis. 
Experimenters  needed  the  data  to  evaluate 
solar  wind  and  extreme  solar  ultraviolet 
radiation  conditions  in  the  planet's  atmosphere 
and  ionosphere.  However,  these  measurements 
had  lower  priority  than  periapsis  science. 

Orbital  Geometry  and  Spacecraft 

Because  Venus  is  so  close  to  the  Sun,  the  Sun's 
gravitational  pull  noticeably  perturbed  the 
spacecraft's  inclined  orbit.  The  significant 
changes  were  in  the  altitude  and  latitude  of 
the  periapsis.  For  the  first  20  months  of  the 
mission,  the  spacecraft  used  its  thrusters  to 
counteract  these  effects.  This  allowed  it  to 
maintain  the  periapsis  at  a  low  altitude.  After- 
ward, controllers  allowed  the  periapsis  to  rise 
by  solar  perturbation. 

Project  management  logically  divided  the  mis- 
sion into  three  phases.  (See  previous  sections 
for  more  details  on  these  three  phases).  Phase  I 

covered  the  period  when  the  thrusters  main- 
tained periapsis  at  an  altitude  of  150  to  250  km 
(93  to  155  miles).  Phase  II  started  when  project 
managers  decided  that  they  should  no  longer 
control  periapsis.  Rather,  they  would  allow  it 
to  rise  and  later  to  fall  under  the  influence  of 
solar  perturbations.  This  conserved  hydrazine 
propellant  for  use  in  extending  an  entry  phase 
(Phase  III).  During  the  second  phase  of  Orbiter 
operations,  the  periapsis  did  more  than  just 
rise  and  fall.  The  latitude  of  the  periapsis  also 
moved  from  1 7°  north  to  the  equator  of  Venus. 
During  this  period,  measurements  at  periapsis 
were  continually  exploring  new  regions  of  the 
planet's  environment. 

Late  in  1991,  the  periapsis  began  to  penetrate 
the  lower  thermosphere  and  ionosphere.  When 
it  had  fallen  to  about  1000  km  (621  miles), 
Phase  III  of  the  mission  began.  As  the  periapsis 
continued  to  fall,  controllers  again  used  the 
thrusters  to  maintain  periapsis.  This  time,  they 
kept  it  within  an  altitude  ranging  from  140  to 
160  km  (87  to  100  miles).  Also,  the  latitude  of 
the  periapsis  continued  moving  southward  to 
about  10°  below  the  planet's  equator.  During 
Phase  III,  the  spacecraft  sampled  the  atmosphere 
to  deeper  levels  than  were  prudent  in  Phase  I. 

The  spacecraft's  orbit  was  fixed  in  inertial 
space  as  Venus  revolved  around  the  Sun.  So  its 
orientation  with  respect  to  Earth  and  Sun 
changed  as  the  planets  moved  around  the  Sun. 
The  combined  motions  resulted  in  seasons  of 
eclipses  and  occultations.  During  the  eclipse 
period,  the  spacecraft  was  repeatedly  shadowed 
by  Venus  when  near  periapsis.  These  were 
called  periapsis  eclipses.  Apoapsis  eclipses 
occurred  when  the  spacecraft  passed  through 
Venus'  shadow  close  to  the  spacecraft's  apoap- 
sis.  During  an  occultation,  the  Orbiter  passed 
behind  Venus  as  observed  from  Earth.  Occulta- 
tion studies  allowed  the  spacecraft  to  probe 
the  atmosphere  and  ionosphere.  Scientists 

observed  the  effect  on  radio  waves  passing 
through  those  regions  on  the  way  from  the 
spacecraft  to  Earth.  When  periapsis  occurred 
during  occultations,  data  remained  in  the 
DSU.  Later,  communications  equipment 
transmitted  the  information  to  Earth. 

As  Venus  traveled  around  the  Sun,  the  planet 
rotated  slowly  under  the  orbit  of  Pioneer.  This 
permitted  the  sub-spacecraft  point  to  pass  over 
the  whole  planet  in  a  period  of  243  days.  This 
amounted  to  the  period  of  one  rotation  of  Venus 
on  its  axis.  However,  the  spacecraft  sampled  all 
local  times  on  Venus  in  its  year  of  224  days. 
That  is,  it  sampled  longitudes  relative  to  the 
Sun  as  contrasted  with  longitudes  on  the 
planet's  surface. 

Flight  Operations 

During  the  Orbiter's  long  mission  at  Venus, 
support  services  contractor  personnel  continued 
routinely  to  conduct  flight  operations.  These 
included  other  Pioneer  missions,  too.  They  per- 
formed their  work  in  the  PMOC  at  Ames 
Research  Center.  The  overall  Pioneer  program 
had  begun  in  the  summer  of  1965.  Since  that 
time,  computers  had  remained  on  and  the 
facility  operated  24  hours  per  day,  365  days 
per  year.  The  only  exception  was  for  one  or 
more  shifts  on  major  national  holidays. 
Console  operators  maintained  constant  voice 
communications  with  the  operations  center  of 
DSN  at  the  Jet  Propulsion  Laboratory,  Pasa- 
dena, California.  These  operators  handled  all 
immediate  detailed  coordination  of  tracking 
operations,  command  transmissions,  and 
telemetry  data  flow.  At  least  one  flight  opera- 
tor and  a  computer  operator  were  nominally 
on  duty  at  all  times  at  Ames  Research  Center 
for  the  extended  Pioneer  missions. 

Daily  operations  usually  included  one  or  two 
passes  for  Pioneer  10  and  Pioneer  11.  These 
were  the  first  spacecraft  to  explore  the  outer 



Solar  System  and  beyond.  Daily  operations 
also  included  an  occasional  (two  or  three  per 
year)  pass  for  one  of  the  Pioneer  6-8  series. 
These  spacecraft  had  first  explored  the  inter- 
planetary environment  and  the  effects  of  solar 
activity  on  Earth.  Each  pass  was  typically  4  to 
11  hours  in  duration.  The  operating  schedules 
of  DSN  imposed  exceptions.  Occasional  special 
computing  circumstances  also  created  excep- 
tions to  these  durations. 

The  engineering  staff  of  Ames  Research  Center 
defined  and  documented  operations  procedures. 
PC-488,  Pioneer  Venus  Orbiter  In-Orbit  Operations, 
specified  operations  for  the  Pioneer  Venus 
Orbiter.  PC-250,  Pioneer  F/G  Standard  Procedures 
for  Flight  Operations,  specified  operations  for 
Pioneers  10  and  11.  PC-053,  Pioneer  A  Flight 
Operations — Standard  Procedures,  defined  the 
operating  procedures  for  Pioneer  6.  It  also  was 
applicable  for  Pioneers  7  and  8.  However,  the 
DSN  Network  Operations  Plan  616-55  was  a 
more  current  reference  for  Pioneers  6-8. 

Staff  specialists  at  Ames  Research  Center  or 
the  contractor  also  attended  special  operations 
These  included  maneuvers  or  unusual  space- 
craft or  instrument  tests.  A  specially  qualified 
contractor  representative  directed  the  more 
routine  procedures.  This  person  regularly 
reviewed  plans  and  procedures  with  engineers 
at  the  Center. 

Duty  personnel  had  lists  of  home  telephone 
numbers  for  engineers.  They  called  these 
engineers  when  prescribed  procedures  could 
not  resolve  problems.  If  telephone  communi- 
cation was  inadequate,  engineers  usually  could 
be  in  the  Center  within  about  30  minutes.  As 
experience  grew  during  the  mission,  the  worst 
of  these  types  of  problems  diminished.  There 
was  an  average  of  several  months  between 
occurrences.  Former  Pioneer  team  members 

experienced  in  the  spacecraft's  development 
also  offered  their  advice  on  request. 

Maintenance  of  computers  used  in  Pioneer 
operations  was  contracted  to  companies 
specializing  in  computers.  If  any  one  computer 
was  down,  the  facility  still  had  sufficient  depth 
to  continue  working  with  any  two  spacecraft. 
This  minimized  off-hour  premium  mainte- 
nance expenses.  A  Pioneer  Missions  Office 
staff  engineer  monitored  and  directed  the 
maintenance  contract  support. 

The  Navigation  Team  at  the  Jet  Propulsion 
Laboratory  analyzed  and  processed  metric 
tracking  data.  This  team  provided  trajectory 
predictions  to  DSN.  Navigators  needed  the 
predictions  for  computations  of  pointing 
angles  and  Doppler  shifts  that  they  used  in 
operations  with  all  Pioneer  spacecraft.  The 
Navigation  Team  also  provided  periodic  pre- 
dictions of  trajectory  parameters  for  computer 
use  at  Ames  Research  Center. 

Planning  and  Development 

Throughout  the  mission's  several  phases,  the 
staff  of  the  Ames  Research  Center  Pioneer 
Missions  Office  provided  general  plans  and 
prepared  procedures.  They  worked  under  the 
coordination  of  the  Flight  Director.  The  sup- 
port service  contractor  at  Ames  Research 
Center  was  Bendix  Field  Engineering  Corpora- 
tion. This  group  translated  the  general  plans 
into  detailed  schedules  for  command  trans- 
missions and  real-time  data  communications 
and  processing. 

DSN  produced  both  long-term  and  near-term 
weekly  schedules  for  project  support  during  the 
extended  mission.  It  also  had  done  this  during 
Phase  I.  At  Ames  Research  Center,  the  support 
service  contractor  produced  a  detailed  weekly 
computer  schedule  for  telemetry  and  command 

activities.  DSN  resources  needed  to  support  the 
Pioneer  spacecraft  were  scheduled  through  the 
Pioneer  Missions  representative  on  the  Jet  Pro- 
pulsion Laboratory  staff.  This  person  main- 
tained close  liaison  with  the  Pioneer  Missions 
Office.  The  representative  adjusted  plans  to 
tracking  availability  and  considered  constraints 
and  special  requirements  of  the  Pioneer  missions 
in  the  scheduling  process.  The  representative 
also  assisted  in  negotiating  with  other  users  of 
DSN  to  resolve  conflicting  requirements. 

The  Pioneer  Missions  Office  and  support  con- 
tractor also  maintained  lists  of  software  and 
hardware  problems.  For  continuing  and 
improving  operations  during  later  phases  of 
the  mission,  they  had  to  resolve  these  prob- 
lems. Most  of  the  modest  developmental  effort 
was  dedicated  to  solving  relatively  short-term 
problems.  These  arose  as  circumstances 
changed  or  as  long-standing  complaints  were 
solved.  Long-term,  larger  projects  for  the  space- 
craft mission  were  also  worked  on  to  maintain 
compatibility  with  DSN's  computer  interfaces. 
These  included  DSN  commands  and  bit  error 
correction  and  longer  data  blocks  for  NASCOM. 
Maintaining  very  long  term  competence  in  the 
data  processing  software  was  an  important 
objective  in  scheduling  these  efforts. 


During  all  phases  of  the  Orbiter's  mission, 
SSG  meetings  occurred  semiannually.  Orbital 
Mission  Operations  Planning  (OMOP)  com- 
mittee meetings  generally  took  place  concur- 
rently with  the  SSG  meetings.  The  committees 
used  teleconferencing  when  it  was  necessary. 
These  groups  recommended  allocation  of  the 
limited  telemetry  data  link  among  the  various 
scientific  interests.  These  interests  included  the 
alignments  of  the  Sun/Venus/orbital  plane,  the 
available  time  for  tracking,  and  other  con- 
straints. The  SSG  meetings  provided  a  general 

exchange  of  information  about  scientific 
progress,  planning  for  publications,  and 
special  interdisciplinary  investigations.  The 
OMOP  committee  resolved  problems  that  were 
more  frequent  and  immediate  than  the  issues 
SSG  addressed.  Individual  investigators  also 
provided  regular  and  frequent  instructions 
about  the  configuration  to  be  commanded  to 
their  instruments. 

Toward  the  end  of  the  Orbiter's  long  mission, 
the  SSG  gave  special  attention  to  the  available 
hydrazine  propellant.  They  carefully  planned 
its  use  to  maintain  the  spacecraft's  orbit  and  to 
optimize  the  return  of  scientific  data.  At  the 
SSG  meeting  in  Spring  1989,  an  Operations 
Plan  Task  Force  (OPTF)  formed.  Its  charter  was 
to  describe  the  scientific  rationale  for  Phase  III. 
To  meet  the  science  goals,  it  also  defined 
requirements  for  experiment  operations, 
science  sequences,  formats,  data  storage,  bit 
rates,  and  quick-look  and  post-entry  data 
analysis.  The  Task  Force  submitted  its  plan  at 
the  SSG's  Spring  1990  meeting.  The  group  did 
not  immediately  settle  many  of  the  plan's  final 
details.  They  waited  until  after  the  SSG  made 
decisions  on  priorities  for  competing  scientific 
goals  and  instrument  operations  during  the 
entry  period.  Many  issues  were  involved. 
Among  them  were  the  collection  of  additional 
radar  altimeter  measurements,  the  relative 
priority  of  low-altitude  in  sitii  measurements, 
the  importance  of  drag  measurements,  and 
questions  of  spin  rate  and  spin  axis  orienta- 
tion. These  were  all  crucial  to  defining  orbital 
sequences,  instrument  modes,  and  intervals 
between  periapsis  restoration  adjustments. 

Until  the  committees  resolved  these  questions 
of  priority,  the  OPTF  followed  a  specific 
course.  It  based  its  plan  on  inputs  that  experi- 
menters had  given  to  the  Entry  Planning 
Committee  at  earlier  SSG  meetings.  The  Task 



Force  was  particularly  interested  in  data  that 
experimenters  had  presented  at  the  1988 
Spring  meeting  in  Annapolis.  The  committees 
later  distributed  this  information  to  the  SSG 
membership.  These  inputs  were  the  most 
complete  at  that  date.  They  were  adequate 
enough  to  define  the  long  lead-time  items  the 
Task  Force  needed  to  support  the  plan.  Such 
items  included  quick-look  data  requirements, 
spacecraft  spin  rate,  and  spacecraft  orientation. 
Also  included  were  changes  to  instrument  data 
processing  software  that  were  required  by 
proposed  changes  in  data  formats. 

Status  After  a  Decade  in  Orbit 

The  spacecraft  design  featured  flexibility  and 
redundancy.  Controllers  could  select  either  of 
two  electrical  components  for  nearly  all  critical 
functions.  These  functions  were  receiving  com- 
mands, storing  and  executing  commands  over 
an  extended  interval  of  time,  processing  data, 
transmitting  telemetered  data,  storing  and 
replaying  telemetered  data,  controlling  despin 
of  the  antenna  and  other  spin-synchronous 
functions,  and  firing  thrusters.  Also,  the  design 
featured  backups  for  the  despin  motor  for  the 
antenna,  the  liquid-propellant  thrusters,  and 
the  electrical  storage  batteries. 

By  1988,  after  10  years  in  orbit,  the  Orbit er 
spacecraft  continued  in  excellent  working 
order.  The  conservative  design  and  redundancy 
of  critical  subsystems  had  paid  off  admirably. 
All  functions  were  serviceable  with  only 
modest  degradations  (when  compared  with 
conditions  immediately  after  entering  orbit 
around  Venus).  For  example,  the  amount  of 
power  the  solar  panels  produced  had  dimin- 
ished during  the  extended  mission.  This 
occurred  because  they  had  not  been  designed 
for  so  many  years  exposure  to  solar  radiation 
at  the  distance  of  Venus  from  the  Sun.  Origi- 
nally, mission  scientists  had  intended  the 
spacecraft  to  orbit  Venus  gathering  data  for 

one  Venusian  sidereal  day,  or  243  Earth  days. 
That  was  the  approved  primary  and  nominal 
mission.  However,  designers  believed  there 
could  be  an  extended  mission  of  gathering 
data  for  several  Venusian  sidereal  days.  The 
conservative  design  allowed  the  spacecraft  to 
eventually  operate  beyond  a  complete  solar 
cycle  of  1 1  years.  This  feat  provided  a  cost- 
effective  bonus  of  scientific  data. 

Figure  5-18  shows  how  solar  activity  was  high 
during  Phase  I  when  navigators  maintained 
periapsis  at  a  low  altitude.  When  periapsis  had 
reached  its  highest  altitude  during  Phase  II, 
solar  activity  was  at  a  minimum.  However,  it 
increased  as  the  periapsis  descended  again. 
During  another  period  of  low  periapsis  in 
Phase  III,  solar  activity  had  passed  through 
its  maximum  and  had  decreased  to  an  inter- 
mediate level. 

Current  from  the  solar  cells  had  decreased 
from  an  average  of  13  A  at  orbit  insertion  to 
4  A.  As  a  result,  power  production  from  the 
solar  cells  limited  operations  to  less  than 
24  hours  per  day.  Fortunately,  the  design 
provided  for  regular  battery  operations  supple- 
menting the  solar  cells.  This  provision  sup- 
ported intermittent  loads.  Starting  in  1988, 
scientific  instruments  that  operated  for  long 
hours  were  used  in  a  time-sharing  mode  to 
maintain  power  balance. 

Data  collecting  and  handling  for  all  the 
science  experiments  was  still  normal  after 
10  years  in  space.  However,  the  failure  of  one 
unit  reduced  storage  and  replay  capacity. 
Telemetry  continued  normal.  One  transmitter 
had  slightly  diminished  power,  but  worked 
with  the  same  efficiency.  The  command 
receiving,  decoding,  storage,  and  executive 
systems  continued  to  perform  perfectly.  How- 
ever, the  secondary  receiver  responded  to  only 
a  narrow  radio  frequency  band.  The  control 

systems  for  spin  axis  orientation,  spin  rate 
control,  antenna  despin  and  elevation  control, 
and  synchronization  timing  signals  all 
worked  perfectly. 

During  the  first  decade  in  orbit,  only  four 
random  failures  occurred  in  the  spacecraft's 
subsystems.  There  were  no  complete  failures  of 
critical  components.  All  the  critical  subsystems 
still  had  serviceable  backups  to  continue  the 
extended  mission  toward  Phase  III.  At  that 
time,  navigators  would  control  the  periapsis 
before  the  spacecraft  finally  plunged  into  the 
Venusian  atmosphere. 

Hydrazine  propellant  consumption  had  been 
conservative  relative  to  pre-launch  estimates. 
The  spacecraft  entered  orbit  around  Venus 
with  32  kg  (70  Ib)  of  propellant.  An  estimated 
2.3  kg  (5  Ib)  remained  following  Phase  I 
operations  to  keep  the  periapsis  at  a  low  level. 
Although  the  precise  amount  of  remaining 
propellant  was  uncertain,  project  management 
estimated  that  enough  remained  for  Phase  III. 
There  was  sufficient  fuel  to  control  the  entry 
sequence  at  the  end  of  the  mission  sometime 
in  1992.  There  also  was  a  possibility  of  control- 
ling the  spacecraft  until  final  entry  into  the 
atmosphere  on  the  planet's  dayside. 

There  were,  of  course,  other  hazards.  Although 
the  spacecraft  had  survived  far  beyond  its 
original  design  lifetime,  there  was  always  the 
possibility  that  some  critical  component  might 
suddenly  fail.  If  this  happened  during  this 
final  phase  of  the  mission,  it  could  bring  the 
project  to  a  premature  end.  Nevertheless, 
project  management  optimistically  expected 
that  they  could  control  the  Orbiter  for  20  to 
40  passes  through  Venus'  atmosphere  before 
atmospheric  forces  during  entry  destroyed  it. 
This  was  very  important.  It  would  provide  a 
unique  opportunity  to  make  measurements  in 
much  lower  regions  of  Venus'  atmosphere. 

These  were  regions  where  no  other  spacecraft 
had  made  measurements  and  where  no 
planned  spacecraft,  such  as  Magellan,  would 
be  designed  to  do  so. 

Hardware  Status  as  the  Final 
Phase  Approached 

The  spacecraft's  attitude  control  system 
operated  successfully  throughout  the  mission. 
The  Despin  Control  Electronics  (DCE)  con- 
sisted of  a  primary  and  a  backup.  Controllers 
switched  off  the  primary  in  1984  and  used  the 
backup  from  that  time.  Although  there  were 
no  problems  with  the  primary  system,  the 
complexity  of  switching  back  to  it  encouraged 
continued  use  of  the  secondary  system.  The 
star  sensor  had  duplicate  slits  and  electronics. 
Both  were  used  successfully  without  any 
problems.  However,  solar  protons  affected  the 
star  sensors.  As  a  result,  the  mission  never  used 
them  during  solar  proton  events  that  lasted 
more  than  several  hours.  Starting  in  December 
1990,  controllers  powered  off  both  star  sensors 
except  when  they  had  to  check  attitude.  This 
reduced  the  load  on  the  spacecraft's  batteries. 

The  spacecraft's  propulsion  system  performed 
well  throughout  the  mission.  All  seven  thrust- 
ers  continued  to  operate  normally.  However, 
one  did  show  a  decrease  in  performance,  so 
the  mission  did  not  use  it  after  October  1984. 

One  DSU  failed  in  March  1986,  and  the 
mission  did  not  use  it  after  that  date.  The 
second  unit  continued  to  operate  throughout 
the  mission.  It  had  an  operational  restriction: 
the  maximum  bit  rate  the  instrument  could 
store  was  2048  bits/sec. 

The  command  subsystem  worked  perfectly 
throughout  the  mission.  However,  there  were 
minor  problems  in  the  communications  sub- 
system. The  spacecraft  carried  two  receivers.  Of 
the  two,  the  backup,  connected  to  the  aft  omni 






2  400 






E-  300 







S.  200 




Local  time 


|an           Feb          Mar          Apr          May         |un           Jul          Aug         Sep          Oct         Nov         Dec          Jan 



I               I 

1             1 


1                    1 

1                    1 

|^-  Final 


encounter  acti 

1                    1 


Subsolar  ionopause 
Magnetic  barrier 
Hot  oxygen 
Plasma  wave  heating 
Thermosphere  dynamics 
Thermosphere  energetics 


;ep  cryosphere 
osphere  energ 
osphere  dyna 
mside  ionosph 
inetic  properti 


Ion  acce 


rmal  ions 
al  electrons 





Central  night 

side  heating 
ion  drift 

Ion  heat  source 
Ionospheric  holes 
Upward  ion  drift 



ird  ion  flow 
inator  waves 

\                 Nightward  ion  flow 
\             Post-terminator  waves 
\                  Gravity  waves 


ty  waves 
3  hazes 



Limb  hazes 



I          i 

08         10 




1          l          1 
N          14        16 



ivity  anomalies 


i         i         i 

10          N         14 




i          i          i 

20        22         M 

i          i          i 

02         04       06 

i          i          i 
18        20        22 

i         i 

M         02 

i         i          i 

|04        06        08 

50                   4800                  4850                  4900                   4950                   5000                   5050                  5100                   5150 
Orbit  number 


Figure  5- 1 8.  This  calendar 
plots  the  altitude  of  the 
periapsis  during  1 992 
against  orbit  number 
(bottom  scale)  and  date 
(top  scale).  Captions  identify 
the  periods  when  the 
spacecraft  investigated 
various  lower  altitude 

antenna,  developed  a  minor  problem  in 
April  1983.  The  uplink  bandwidth  became 
restricted  to  about  250  Hz  for  signal  detection 
and  about  50  Hz  for  command  processing. 
However,  DSN  developed  a  special  procedure 
to  allow  that  receiver  to  process  at  least  one 
command  if  needed.  The  primary  receiver, 
connected  to  the  high-gain  antenna, 
performed  well  throughout  the  mission. 

There  were  four  amplifiers  available  to  power 
the  downlink  transmission  to  Earth.  Since 
February  1984,  controllers  had  turned  off 
one  amplifier  because  of  a  limitation  on 

power.  In  May  1984,  the  output  of  another 
amplifier  dropped  about  one  decibel  and 
became  noisy.  Mission  controllers  switched  it 
off.  The  remaining  two  amplifiers  did  not 
develop  any  problems. 

There  were  no  failures  in  the  spacecraft's 
power  subsystem.  However,  both  the  solar 
arrays  and  the  nickel-cadmium  batteries 
degraded  during  the  mission,  as  expected. 
Since  the  three  solar  arrays  were  electrically 
independent,  solar  radiation  darkening  the 
glass  covering  the  solar  cells  probably  caused 
their  degradation.  To  compensate  for  the 

reduced  power  from  the  solar  arrays,  the  space- 
craft had  to  use  its  batteries  more  frequently. 
However,  their  recharging  took  longer  because 
of  the  reduced  output  from  the  solar  arrays. 
The  two  batteries  provided  power  for  the 
spacecraft  when  it  was  in  the  shadow  of  Venus 
(eclipse  periods).  The  spacecraft  also  used  them 
to  supplement  power  from  the  solar  arrays  when 
the  electrical  load  exceeded  the  output  from 
the  arrays.  As  the  solar  array  output  declined, 
controllers  had  to  use  the  batteries  more  and 
more.  Also,  use  of  the  batteries  once  for  each 
spin  of  the  spacecraft  slowly  degraded  their 
capacity.  When  the  output  voltage  dropped  to 
27.5  V,  an  undervoltage  switch  turned  off  all 
nonessential  systems  to  protect  the  batteries. 

Toward  the  end  of  the  mission,  both  the  solar 
arrays'  output  and  the  batteries'  capacity  some- 
times varied  without  warning.  So,  mission  con- 
trollers had  to  balance  energy  usage  daily.  This 
was  necessary  for  each  battery  because  each  had 
its  own  solar  array.  The  procedure  was  to 
recharge  each  battery  completely  on  each  orbit 
of  the  spacecraft.  This  was  done  just  before  peri- 
apsis  tracking  or  before  an  eclipse,  and  when  the 
spacecraft  was  not  using  the  transmitter  ampli- 
fier. Controllers  had  to  switch  off  other  equip- 
ment during  the  charging  period.  However, 
mission  controllers  had  to  ensure  that  the  bat- 
teries did  not  overheat  by  overcharging.  They 
carefully  monitored  and  evaluated  battery  loads. 
They  calculated  output  of  the  solar  arrays  and 
organized  everything  to  maintain  battery  voltage 
at  more  than  28.5  V.  This  was  one  volt  above 
the  level  at  which  the  undervoltage  switch 
would  switch  off  all  nonessential  systems. 

An  anomaly  with  the  magnetometer  boom  did 
not  affect  operations.  Telemetry  signals 
indicated  that  the  boom  had  not  deployed. 
However,  performance  of  instruments  on  the 
boom  indicated  that  it  was  fully  deployed  and 
properly  locked  into  place. 

An  Opportunity  to  Look  at  Comets 

During  Phase  II  of  Orbiter's  mission,  opportu- 
nities arose  to  make  systematic  observations  of 
several  comets.  To  do  this,  controllers  used  the 
Orbiter  Ultraviolet  Spectrometer  (OUVS). 
These  observations  took  place  between  April 

1984  and  May  1987.  The  comets  and  their 
dates  of  observation  were:  Encke,  April  13 
through  16,  1984;  Giacobini-Zinner,  Septem- 
ber 8  through  15,  1985;  Halley,  December  27, 

1985  to  March  9,  1986;  Wilson,  March  13  to 
May  2,  1987;  NTT,  April  8,  1987;  and 
McNaught,  November  19  through  24,  1987. 

These  observations  had  several  scientific 
objectives.  They  included  determining  the 
evolution  rate  of  water  from  the  cometary 
nucleus  and  how  it  varied.  Identifying  the 
carbon/oxygen/hydrogen  ratios  and  how  they 
varied  was  another  objective.  Also,  the  obser- 
vations looked  for  evidence  of  rotation  of  the 
comet's  nucleus.  Experimenters  used  the 
spectrometer  to  obtain  images  of  the  coma  in 
Lyman-alpha  radiation. 

Jim  Phillips  was  Pioneer  Venus  Project  Trajec- 
tory Analyst.  He  explained  that  the  comet  mis- 
sions were  accomplished  by  sending  commands 
to  the  spinning  spacecraft  to  change  the  tilt  of 
its  spin  axis.  This  allowed  the  spectrometer  to 
scan  across  the  comet  instead  of  across  the 
surface  of  Venus.  The  ultraviolet  spectrometer 
had  a  small  field  of  view.  As  a  result,  it  gath- 
ered general  pictures  of  the  comets  as  a  series 
of  strips,  one  for  each  spin  of  the  spacecraft. 
The  comet  moved  slightly  between  each  spin 
to  expose  a  sequence  of  strips  along  its  length. 
The  ability  to  scan  at  high  resolution  across 
the  comet's  coma,  tail,  and  hydrogen  halo 
was  important.  It  allowed  scientists  to  deter- 
mine the  distribution  of  gas  and  particles  in 
each  region. 



Unique  observations  of  several  comets  by  one 
spacecraft  benefited  from  using  the  same 
instrument.  This  was  an  important  capability 
for  making  comparisons  among  several  comets 
of  different  ages.  For  example,  Comet  Wilson 
is  a  new  comet  discovered  in  1986.  Comet 
Encke  is  an  evolved  comet,  one  of  the  most 
evolved  comets  known.  Comet  Halley  is  a 
"middle-aged"  comet.  In  the  case  of  Comet 
Halley,  the  Pioneer  Venus  observations  were 
special.  They  gathered  data  about  the  comet 
close  to  the  comet's  perihelion  passage 
(Figure  5-19).  Other  spacecraft  were  not  in 
positions  where  they  could  accomplish  this. 

All  the  comet  encounters  were  successful,  and 
the  results  appear  in  a  later  chapter. 

Final  Encounter 

Earlier  sections  described  how  the  encounter- 
phase  science  plan  gave  periapsis  data  gather- 
ing the  highest  priority.  They  also  outlined 
how  instruments  would  cover  every  periapsis 
passage.  Data  gathering  elsewhere  in  the  orbit 
had  a  lower  priority.  Mission  controllers  would 
not  allow  it  to  compromise  periapsis  data 
gathering.  During  the  period  from  1  Septem- 
ber 1992  to  the  end  of  the  mission  in  October, 
low-altitude  periapsis  passages  through  Venus' 
atmosphere  dominated  spacecraft  activities. 
Velocity  maneuvers  using  the  remaining 
thruster  propellant  were  important.  They 
maintained  the  altitude  of  the  periapsis  above 
a  point  where  drag  would  be  unacceptable  and 
the  orbit  endangered.  Controllers  also  made 
precession  maneuvers.  These  corrected  the 
attitude  shift  that  atmospheric  drag  produced 
and  the  required  attitude  changes  that  the 
velocity  maneuvers  produced  to  maintain 
periapsis  alti-  tude.  Spin  rate  adjustments  were 
also  required  to  correct  the  despin  caused  by 
altitude  control  maneuvers.  During  this 
period,  the  orbit  period  shortened.  As  a  result, 
controllers  had  to  adjust  tracking  sequences 

to  ensure  that  they  obtained  data  during 
periapsis  passage. 

Generally,  DSN  maintained  tracking  periods  at 
periapsis  and  at  apoapsis  centered  about 
12  hours  apart.  However,  during  the  critical 
orbits  approaching  final  entry,  the  spacecraft 
was  in  Earth  occultation  during  the  periapsis 
period.  So,  DSN  could  not  track  the  spacecraft 
or  receive  data  from  it  at  periapsis. 

To  maintain  the  altitude  of  periapsis,  the 
thruster  changed  the  spacecraft's  velocity  at 
apoapsis.  There  were  instances  during  this 
time  when  the  spacecraft  could  not  receive 
commands.  This  was  because  the  undervoltage 
switch  had  switched  off  the  high-gain  antenna 
or  the  despin  control.  This  made  the  prime 
receiver  unavailable.  Without  the  prime 
receiver,  controllers  were  concerned  about  the 
critical  command  capability  that  was  needed 
to  complete  the  periapsis-maintaining  maneu- 
ver on  time.  If  the  maneuver  was  delayed, 
there  was  a  chance  that  the  spacecraft  would 
be  destroyed  during  the  next  periapsis  passage 
because  it  was  too  deep  in  the  atmosphere. 

The  inability  to  receive  a  command  had  caused 
a  delay  of  36  hours  before  controllers  could 
switch  to  another  receiver.  During  earlier 
phases  of  the  mission,  this  was  not  a  serious 
problem.  However,  it  was  not  acceptable 
during  this  final  encounter  when,  even  if  the 
spacecraft  survived  a  lower  periapsis,  all 
periapsides,  12  hours  apart,  had  to  be  covered. 
Moreover,  the  backup  receiver's  performance 
had  degraded.  To  receive  just  a  single  com- 
mand, it  needed  special  operating  procedures 
from  DSN.  DSN's  high-power  (250  kW)  trans- 
mitter was  required  for  supporting  altitude 
raising  maneuvers. 

The  minimum  safe  periapsis  altitude  was 
established  as  being  just  above  where  drag- 


Orbit  of  Comet  Halley 





induced  damage  to  the  spacecraft  would  occur. 
It  varied  as  a  function  of  the  hour  angle  of  the 
periapsis  from  the  sub-solar  point  on  the 
planet.  The  exposed  materials  most  likely  to  be 
damaged  by  drag  heating  were  the  Kapton 
thermal  blankets  and  the  small  fiberglass 
structural  elements  supporting  the  antennas. 
The  minimum  altitude  was  expected  to  be 
131  km  (81  miles)  in  the  dark  and  142  km 
(88  miles)  in  daylight.  This  was  because  the 
density  of  the  atmosphere  at  a  given  height 
varies  considerably  between  day  and  night  on 
the  planet.  Measurements  of  periapsis  altitude 
could  be  made  to  within  a  few  hundred  meters 
by  a  single-pass  Doppler  observation,  and  to 
within  a  few  tens  of  meters  by  Doppler 
observations  over  several  sequential  passages. 

Entry  Phase 

By  early  September  1992,  the  altitude  of  the 
periapsis  was  approaching  a  level  where  it 
could  damage  the  spacecraft.  By  this  time,  the 
remaining  propellant  was  estimated  at  1.86  kg 

(4.1  Ib).  This  propellant  was  to  be  used  to 
extend  the  mission  as  long  as  possible.  The 
sequence  of  periapsis  decay  and  altitude  trim 
maneuvers  during  the  mission's  final  phase 
appears  in  Figure  5-20  (relative  to  the  local  time 
on  Venus). 

Larry  Lasher,  Pioneer  Mission  Science  Chief 
said,  "My  most  memorable  recollection  of 
the  project  occurred  during  the  final  days  of 
Pioneer  Venus  in  early  fall  1992,  observing  the 
excitement  and  enthusiasm  of  the  scientific 
investigators  as  they  made  once-in-a-lifetime 
measurements  as  PVO  [Pioneer  Venus  Orbiter] 
charted  previously  unexplored  regions  of  the 
Venus  atmosphere." 

As  the  figure  shows,  the  first  planned  periapsis- 
raising  maneuver  was  performed  on  Septem- 
ber 7,  1992,  when  periapsis  was  raised  by  20  km 
(12  miles)  to  about  155  km  (96  miles).  This  left 
sufficient  propellant  for  five  more  maneuvers 
to  maintain  the  altitude  of  periapsis.  During 

Figure  5- 1 9.  This  diagram  shows 
how  the  Pioneer  Venus  Orbiter 
was  in  an  advantageous  position 
to  observe  Comet  Halley  around 
the  important  time  of  its 
perihelion  passage.  European, 
Soviet,  and  Japanese  spacecraft 
sent  to  fly  by  the  comet  were  not 
able  to  encounter  the  comet  and 
make  their  observations  at  its 


Figure  5-20.  This  figure 
illustrates  how,  during  the 
entry  phase,  periapsis 
repeatedly  allowed  the  space- 
craft to  scan  the  very  low 
thermosphere.  This  region  is 
where  differences  in  the  neutral 
mass  spectrometer  and  drag 
measurements  occurred.  The 
differences  resulted  from  opera- 
tion below  the  region  of  free 
molecular  flow.  Aerodynamic 
heating  and  impact  ionization 
affected  the  accuracy  of  some 
measurements.  In  the  top  right 
quadrant,  the  periapsis  is 
raised  several  times  by  use 
of  the  spacecraft's  thrusters. 
The  bottom  right  quadrant 
illustrates  a  hoped-for  path 
for  entry  into  the  sunlit 
hemisphere  without  having 
to  use  thrusters.  Unfortu- 
nately, there  was  not  enough 
propellant  left  for  this  final 
maneuver  and  entry  took 
place  in  the  night  hemisphere 
in  the  upper  right  quadrant. 


the  maneuver  executed  on  October  2,  the 
spacecraft  used  the  last  of  the  propellant. 
As  a  result,  the  periapsis  dropped  lower  and 
lower  into  the  atmosphere.  The  spacecraft 
could  not  be  tracked  during  the  periapsis,  nor 
could  data  be  received  from  it.  At  the  control 
center,  everyone  had  to  wait  until  the  space- 
craft's signal  could  be  acquired  again  when  the 
Orbiter  moved  out  from  Earth  occultation. 

Said  Lasher:  "There  was .  . .  the  expectation 
and  hope  of  the  project  staff  huddled  around 
the  consoles  in  mission  control  listening  for 
the  spacecraft  signal,  hoping  for  just  one  more 
orbit  that  fateful  day  . . .  ." 

After  periapsis  passage  of  orbit  5056  on 
October  8,  DSN  stations  received  no  radio 
signals  from  the  Orbiter  when  they  should 

have  at  20:22  hours  GMT.  Nor  did  anyone 
hear  anything  further  from  the  Orbiter. 

The  Pioneer  Venus  Orbiter  had  ended  in  a 
veritable  blaze  of  glory,  as  a  meteor  flaming 
through  the  dense  atmosphere  of  Venus 
(Figure  5-21).  The  spacecraft  had  orbited  the 
planet  since  December  4,  1978,  after  its  launch 
in  May  1978.  It  had  completed  5055  successful 
data  gathering  orbits  of  Venus. 

The  Pioneer-Venus  orbital  mission  was 
declared  completed  on  October  9,  1992,  at 
00:55  GMT.  Said  Lasher,  ".  .  .  we  lost  contact 
forever  with  PVO.  But  I  recall  there  was  a 
certain  serenity  afterward  in  the  knowledge 
that  Pioneer  had  performed  so  nobly  for 
14  years,  far  and  above  its  call  of  duty." 

The  Pioneer-Venus  project  was  a  14-year 
mission  that  far  exceeded  all  its  original 
objectives.  It  became  a  classic  example  of  how 
management  and  science  can  design  and  run 
an  advanced  technology  project  to  achieve 
stated  engineering  and  scientific  objectives  on 
time  and  within  a  clearly  defined  budget. 
Commented  Fred  Wirth,  Pioneer  Deputy 
Project  Manager,  "The  14-year  flood  of  science 
data  from  Pioneer  Venus  has  been  particularly 
rewarding.  The  radar  map  of  the  Venus 
surface,  the  characterization  of  the  bow  shock 
and  the  ionosphere,  pictures  of  the  cloud 
cover,  and  the  glowing  ultraviolet  image  of 
Comet  Halley  somehow  made  it  all  worth- 
while. The  Orbiter  may  be  dead,  but  the  legacy 
of  scientific  data  it  leaves  behind  will  continue 
to  nourish  mankind  in  its  quest  for  knowledge 
for  many  years  to  come." 

Figure  5-21.  As  the  Pioneer 
Venus  Orbiter  entered  the 
Venusian  atmosphere,  it 
produced  a  glowing  trail  like 
a  large  meteorite.  This  artist's 
rendering  shows  the  spec- 
tacular end  of  the  Orbiter's 
1 4-year  mission. 



Photo  Credit  ©Hughes  Aircraft  Company 


At  the  beginning  of 
this  century,  E.  W. 
Maunder  of  the 
British  Royal  Observa- 
tory at  Greenwich 
commented  on  the 
Venus  puzzle.  He  wrote: 
"We  never  see  her  surface; 
she  presents  but  a  dazzling 
disc,  with  never  a  marking 
that  we  can  be  certain  is  not 
the  result  of  eyes  tired  with 
too  much  brightness.  Whether 
her  atmosphere  is  clear  or 
cloudy,  or  what  lies  behind  that 
dazzling  light,  we  do  not  know." 

Fifty  years  passed.  Despite  enor- 
mous increases  in  telescopic 
capabilities,  Venus  still  remained  a 
mystery.  In  1959  and  1961,  just  prior 
to  the  blossoming  of  the  space  age, 
speakers  at  Lunar  and  Planetary 
Exploration  colloquia  summarized  the 
problem.  Commented  W.  E.  Straly:  "As 
opposed  to  the  volume  of  material 
known  about  Mars,  there  is  little  known 
about  Venus.  Its  diameter  is  estimated  at 
0.95  that  of  Earth,  but  this  figure  is  far  from 
exact.  There  is  no  apparent  flattening  of  the 
sphere.  No  surface  features  have  ever  been 
discerned,  its  period  of  rotation  is  indetermi- 
nate, and  little  is  known  of  its  atmosphere.  Its 
polar  caps  are  ill-defined,  and  no  other 
permanent  markings  have  been  seen.  Its 
surface  temperature  has  been  estimated  at 
110°F,  and  surface  pressure  at  two  Earth 
atmospheres,  but  these  are  no  more  than 
educated  guesses." 

In  November  1961,  C.  E.  Anderson  stated:  "The 
rate  of  rotation  of  Venus  is  still  a  problem.  On 

the  basis  of  the  Doppler  measurements,  JPL 
claims  a  period  of  about  225  days.  However,  a 
recent  article  in  Izvestia  stated  a  period  of  10  or 
1 1  days  was  calculated  from  the  Russian  Dop- 
pler measurements." 

Not  until  the  second  decade  of  the  space  age 
did  the  veils  of  mystery  surrounding  Venus 
begin  to  lift.  Scientists  now  were  gaining 
insights  into  the  planet's  true  nature  for  the 
first  time.  This  new  understanding  resulted 
from  experiments  on  the  Pioneer  spacecraft 
and  measurements  from  earlier  flyby  space- 
craft and  from  Soviet  probes  and  orbiters. 

Observing  Venus  at  close  quarters  for  over 
14  years,  Pioneer  Venus  revealed  a  bizarre 
world— a  planet  whose  surface  bakes  under  a 
dense  atmosphere  of  carbon  dioxide  beneath 
clouds  of  sulfuric  acid.  Lack  of  an  intrinsic 
magnetic  field  exposes  the  planet's  upper 
atmosphere  to  the  onslaught  of  the  solar  wind. 
Pioneer  gathered  voluminous  data  about  the 
composition  and  dynamics  of  Venus'  atmo- 
sphere and  nearby  interplanetary  environment. 
It  also  provided  an  initial  radar  exploration  of 
surface  features.  Later,  toward  the  end  of 
Pioneer's  mission,  NASA's  Magellan  spacecraft 
provided  high  resolution  radar  images  of 
Venus'  surface.  These  images  enabled  scientists 
to  study  in  great  detail  the  planet's  geology 
and  internal  structure  and  probable  evolution. 
Some  of  the  Pioneer  radar  data  supplemented 
the  Magellan  data  or  filled  in  parts  of  the 
surface  where  Magellan  data  were  missing. 
Topographers  also  applied  some  Soviet  data  to 
aid  the  global  mapping. 

This  chapter  is  the  longest  in 
this  book.  During  its 
14-year  span,  the  Pioneer 
Venus  mission  collected  a 
wealth  of  data.  Analyzing 
these  data  kept  scientists 
busy  during  the  mission  and 
for  years  after  Orbiter  sent 
its  last  signal  to  Earth.  In 
this  chapter,  you  learn  about 
their  many  discoveries. 
These  include  details  about 
Venus'  surface,  atmosphere, 
clouds,  solar-wind  interac- 
tion, magnetic  fields,  history, 
and  other  discoveries  about 
Earth's  sister  planet. 



The  Planet  in  General 

Radar  data  from  Pioneer  Venus  provided  the 
first  global  elevation  survey  of  Venus'  surface, 
from  which  about  90%  of  the  planet  was 
mapped  topographically.  Before  the  mission, 
Venus'  surface  was  the  least  known  surface  of 
all  the  terrestrial  planets.  Optical  telescopes 
cannot  penetrate  the  clouds,  and  radar  images 
from  Earth  have  limited  resolution  and 
coverage.  When  Venus  is  closest  to  Earth,  it 
rotates  so  it  turns  almost  the  same  hemisphere 
toward  us.  Consequently,  Earth-based  radar 
can  scan  less  than  half  of  the  planet's  surface 
and  only  on  a  narrow  equatorial  swath.  By 
contrast,  the  Orbiter  spacecraft  traveled  in  an 
orbit  allowing  coverage  of  most  of  the  surface. 
Altimeter  mapping  sequences  occupied  about 
1  hour  of  each  orbit  at  altitudes  below 
4700  km  (2921  miles).  The  radar  data  showed 
surface  features  as  small  as  75  km  (47  miles) 
diameter.  The  smallest  cell  size  was  about 
25  km  (15.5  miles),  but  two  or  three  were 
needed  to  define  a  feature  other  than  a  long 
narrow  one  such  as  a  rift.  As  the  orbit  precessed 
around  the  planet,  the  radar  view  gradually 
covered  nearly  all  the  surface.  However, 
resolution  was  reduced  at  high  latitudes. 
Pioneer's  altimetric  sightings  covered  more 
than  90%  of  the  planet's  surface,  from  73° 
north  to  63°  south  latitude  (Figure  6-1). 

To  map  Venus,  the  distance  from  the  space- 
craft to  the  surface  was  measured  by  the  radar 
altimeter.  Since  the  spacecraft's  orbit  was 
accurately  known  from  ground  tracking,  this 
allowed  researchers  to  convert  altitude  mea- 
surements to  radius  measurements  at  discrete 
positions  on  the  surface. 

Pioneer  made  important  discoveries  about 
Venus'  surface.  At  a  scale  at  or  above  100  km 
(62  miles),  Venus  is  mostly  smoother  than  the 
other  terrestrial  planets.  Yet  its  surface  topog- 
raphy has  about  as  much  maximum  positive 

relief  as  Earth's.  However,  the  distribution  of 
elevations  differs  markedly  from  that  on  Earth. 
There  is  only  one  mode  in  surface  height  dis- 
tribution rather  than  two.  Both  topography 
and  gravity  suggest  that  although  Venus' 
interior  is  probably  dynamic,  its  tectonic 
evolution  has  not  been  like  Earth's. 

Pioneer  confirmed  that  Venus  is  quite  round, 
very  different  from  the  other  planets  and  from 
the  Moon.  Earth,  for  example,  is  flattened  at 
the  poles  and  bulges  21  km  (13  miles)  at  the 
equator.  The  Moon  has  a  bulge  toward  Earth. 
Mars  bulges,  too,  but  Venus  has  neither  polar 
flattening  nor  an  equatorial  bulge.  Earth  has 
major  variations  between  continents  and 
ocean  basins,  which  cover  30%  and  70%  of 
the  surface,  respectively.  The  mean  levels  of 
terrestrial  continents  and  ocean  floors  are 
separated  by  4.5  km  (2.8  miles).  Mars  also  has 
major  variations  and  the  colossal  uplift  of  the 
Tharsis  region.  By  contrast,  Pioneer  found  that 
Venus  has  a  very  narrow  distribution  of  surface 
elevations.  Twenty  percent  of  the  planet  lies 
within  125  m  (400  ft)  of  the  mean  radius,  and 
60%  lies  within  500  m  (1600  ft)  of  it.  On  the 
scale  of  the  Pioneer  radar  images,  the  planet 
appeared  as  a  monotonous  world.  It  has  only 
a  few  large  continent-sized  areas  and  smaller 
island  areas  rising  above  a  global  plain. 

The  highest  point  on  the  planet  that  Pioneer 
Venus  measured  was  a  summit  in  Maxwell 
Montes,  10.8  km  (6.7  miles)  above  the  mean 
level.  The  lowest  point  is  2.9  km  (1.8  miles) 
below  the  mean  level.  This  area  is  in  a  rift 
valley  located  at  156°  east  longitude  and 
14°  south  latitude.  This  depth  is  similar  to 
that  of  the  Valles  Marineris  on  Mars  (Figure  6-2). 
It  is,  however,  only  one-fifth  the  greatest  depth 
on  Earth  (in  the  Marianas  Trench). 

Before  the  Pioneer  Venus  mission,  knowledge 
of  Venus'  gravity  field  was  scarce.  Scientists 


240°  270°  300°  330°  0°  30°  60°  90°  120°  150°  180°  210°  240° 


i  i  i  r 







240"  270° 




















Contour  interval  1  km 

Figure  6- 1 . 
Cartographers  made 
the  first  topographic 
maps  of  Venus  in  the 
late  1970s  from 
Pioneer  Orbiter  data. 
(Top)  Topographic 
map  of  the  surface 
from  radar  data;  dark 
gray  is  low,  light  gray 
is  high.  (Bottom) 
Contour  map  of  the 
surface  with  contour 
intervals  of  1.0  km 
(0.6  mile).  The 
highest  point  is  the 
summit  of  Maxwell 
Monies  at  about  350° 
east  longitude  and 
60°  north  latitude. 
The  lowest  point  is  in 
the  rift  valley,  Diana 
Chasma,  at  about 
1 60°  east  longitude 
and  10°  south 
latitude.  The  black 
triangles  show  the 
landing  points  of 
Veneras  8,  9,  and  1 0. 
The  black  dots  show 
the  entry  points  of  the 
four  Pioneer  probes. 
The  map  also  shows 
the  names  and 
locations  of  major 
features  that  the  text 


Figure  6-2.  This  drawing 
compares  major  valleys  on 
Venus,  Mars,  and  Earth.  The 
vertical  scale  is  exaggerated  as 
you  can  see  from  the  horizontal 
and  vertical  scales  that  represent 
Earth's  Grand  Canyon.  These 
scales  apply  to  all  the  valleys  in 
the  figure. 


Venus  Rift  Valley 

Valles  Marineris,  Mars 

Rio  Grande  Rift, 
Northern  New  Mexico 


Sea  Level 

Red  Sea  Rift 





Grand  Canyon,  Arizona 



0  400 


Vertical  exaggeration  40:1 

had  derived  estimates  from  the  flybys  of 
Mariner  2  (1969)  and  Mariner  10  (1974). 
Doppler  radio  tracking  of  Orbiter  during  its 
low  altitude  periapsis  periods  gave  the  first 
detailed  gravity  measurements.  These  allowed 
researchers  to  match  gravity  signatures  with 
surface  topography.  Also,  by  processing  long 
periodic  variations  of  the  Orbiter's  mean 
orbital  elements,  estimates  were  obtained  of 
the  harmonic  coefficients  for  Venus'  gravity 
field.  The  data  confirmed  that  the  planet's 
oblateness  is  very  small,  as  was  expected  from 
its  slow  rotation  rate. 

Detailed  gravity  measurements  over  a  signifi- 
cant area  of  Venus  revealed  many  anomalies 
(Figure  6-3).  But  unlike  those  on  the  Moon 
and  Mars,  Venus'  gravity  anomalies  are  rela- 
tively mild  in  amplitude  and  more  like  those 
of  Earth.  Geophysicists  analyzed  the  spectrum 
of  the  harmonic  model  derived  from  these 
Pioneer  data  and  found  that  topographic  con- 
sequences of  the  anomalies  in  Venus'  interior 
density  are  different  from  those  of  Earth. 

On  Venus,  the  anomalies  match  up  with 
topography;  on  Earth,  most  do  not. 

Scientists  concluded  that  major  adjustment  to 
Venus'  crust  has  taken  place  to  reduce  topo- 
graphic effects.  Also,  partial  isostasy  or  general 
equilibrium  of  crustal  masses  now  prevails. 
Later,  high  resolution  radar  images  obtained 
by  NASA's  Magellan  spacecraft  revealed  lava 
flows  over  much  of  the  planet's  surface. 
Magellan  also  provided  more  gravity  data  that 
suggest  isolated  areas  of  upwelling  among  the 
networks  of  downwellings  in  a  convective 
mantle.  The  uniform  distribution  of  impact 
craters  on  Venus  does  not,  however,  support  a 
terrestrial  type  of  plate  tectonics.  There  also 
appear  to  be  zones  of  thick  crust,  associated 
with  tesserae  such  as  Alpha  Regio.  Large 
volcanic  features  such  as  Beta  Regio  are 
probably  located  over  upwellings.  Beta  is  most 
probably  a  young  feature  supported  by  the 
internal  convective  process. 

Pioneer  Orbiter  investigators  plotted  the  ratio 
of  observed  gravity  to  theoretical  gravity  from 
topography  and  found  a  definite  trend  of  an 
increasing  ratio  eastward  from  Western 
Aphrodite  around  the  planet  to  beyond  Beta 
Regio.  They  suggested  a  possible  explanation: 
a  convection  system  moving  eastward  with  its 
trailing  topography  more  concentrated  at 
larger  distances  from  the  most  active  region. 
Some  investigators  debated  whether  Aphrodite 
Terra  was  a  spreading  center  like  Earth's  mid- 
Atlantic  ridge.  Later  data  from  Orbiter  and 
Magellan  seem  to  rule  out  terrestrial-type 
plate  tectonics  from  spreading  centers.  Even 
so,  some  topography  is  suggestive  of  subduc- 
tion,  for  example,  steep  scarps  on  the  northern 
boundary  of  Ishtar  Terra  and  Ovda  Regio. 

Scientists  used  line-of-sight  accelerations  of 
Orbiter  to  deduce  vertical  gravity.  They 
combined  these  observations  with  topography 

-1(0  -ISO 





data  to  find  mass  anomalies  on  the  crust- 
mantle  boundary  and  in  the  upper  levels  of 
the  mantle.  Comparing  these  results  with 
detailed  radar  maps  from  Magellan,  scientists 
were  able  to  link  vigorous  mantle  upwelling 
with  several  "hot  spots"  on  Venus'  surface. 
They  determined  that  mantle  flow  actively 
drives  surface  rifting.  They  also  found  that 
around  some  hot  spots  return  flow  is  distrib- 
uted asymmetrically.  Geology  of  Venus  appears 
to  be  more  directly  linked  to  mantle  convec- 
tion than  is  Earth's  geology.  Possibly  because 
the  upper  mantle  is  very  dry  and  less  viscous 
than  Earth's,  stresses  to  Venus'  lithosphere 
cause  near-surface  failure. 

The  Surface 

Pioneer  Venus  Orbiter  acquired  radar  data  until 
its  periapsis  rose  too  high  during  Phase  II. 
Researchers  carefully  adjusted  Orbital  tracks  for 
the  first  part  of  Phase  II  (extended  mission)  so 

that  new  data  points  lay  between  those  the 
spacecraft  acquired  during  Phase  I  (nominal 
mission).  To  produce  more  complete  morpho- 
logical and  geological  maps  of  the  planet 
(Figure  6-4)  higher  resolution  data  from 
Veneras  15  and  16  (1985-1986)  were  incorpo- 
rated with  the  Pioneer  Venus  data.  In  the  spirit 
of  international  cooperation,  Soviet  scientists 
made  their  data  available  before  publication,  so 
that  U.S.  scientists  could  plan  more  carefully 
NASA's  Magellan  radar  mapper  mission  to 
Venus.  Since  the  Magellan  mission  was  so  suc- 
cessful, Orbiter's  radar  mapper  was  not  reacti- 
vated when  periapsis  returned  to  lower  altitudes. 

The  Orbiter's  "lifting  of  the  veils  of  Venus" 
revealed  a  world  of  great  mountains,  expansive 
plateaus,  enormous  rift  valleys,  and  shallow 
basins.  Scientists  had  already  deduced  from 
Earth-based  radar  some  of  the  features  that 
Pioneer  revealed.  The  wide  range  of  Pioneer's 

Figure  6-3.  By  measuring 
changes  to  the  orbit  of  the 
Orbiter  spacecraft,  scien- 
tists determined  Venus' 
gravity  anomalies.  This 
figure  shows  sixth  degree 
and  lower-order  harmonic 
coefficients  in  milligals. 
Researchers  calculated 
them  at  1 00  km  (60  miles) 
above  the  mean  surface 
during  an  early  phase  of 
the  Pioneer  mission.  The 
relationship  to  major  sur- 
face features  is  clear.  Later 
in  the  mission,  researchers 
made  even  more  precise 
gravity  measurements, 
especially  during  the  final 
phase  when  periapsis  was 
at  its  lowest.  These  gravity 
maps  showed  that  the 
relationship  of  gravity 
anomalies  to  surface 
features  is  different 
for  Venus  and  Earth. 


Figure  6-4.  Geologists  made  use 
of  Pioneer  data  to  create  a  globe 
of  Venus  showing  its  surface 
features  in  detail.  These  photos 
show  this  globe  from  viewpoints 
centered  on  0°,  90°,  180°,  and 
270°  of  longitude.  The  center  of 
the  crater  Eve,  located  in  Alpha 
Reggio,  was  selected  as  the  0° 
point  of  longitude  on  Venus. 


data  confirmed  the  existence  of  these  features 
and  expanded  detailed  coverage  of  the  planet. 
However,  the  new  Pioneer  data  caused  scien- 
tists to  revise  many  of  their  earlier  theories. 
Venus'  crust  has  three  quite  distinct  regions. 
Relatively  ancient  crust  seemed  to  be  at  inter- 
mediate elevations.  Also,  there  were  smooth 
lowland  plains  and  highlands.  Most  of  the 
planet's  ancient  crust — those  parts  of  the  planet 
between  0  and  2  km  (0  and  1.25  miles)  above 
the  mean  radius — may  be  preserved  in  Venus' 
upland  plains.  Venera  8  landed  in  these  regions 
and  its  gamma-ray  experiment  showed  that 
rocks  there  have  uranium,  thorium,  and  radio- 
active potassium  contents  that  are  consistent 
with  a  granitic  composition.  However,  later 

data  showed  that  these  rocks  may  have  a 
different  composition  (see  also  Chapter  7). 

The  Pioneer  Venus  data  revealed  that  most  of 
Venus  (65%  to  70%)  consists  of  upland  rolling 
plains  on  which  circular  dark  features  were 
identified  as  remains  of  large  impact  craters. 
The  circular  features  were  about  500  to  800  km 
(311  to  497  miles)  in  diameter  but  were  very 
shallow— only  200  to  700  m  (650  to  2300  ft) 
deep.  Scientists  attributed  the  shallowness  to 
erosion  or  to  flooding  with  lava  or  windblown 
deposits.  Bright  spots  in  radar  images  of  the 
craters  suggested  central  peaks,  which  were 
later  confirmed  by  Magellan  images  with 
greater  resolution.  There  also  were  small 














(large  as  Australia) 

Beta  Regio 

shield  volcanos,  larger 

than  Hawaii-Midway 


(large  as  Africa) 











Figure  6-5.  This  map  shows 
the  major  continental  areas  of 
Venus  discovered  by  the  Pioneer 
Venus  spacecraft.  Because  of 
the  Mercator  projection,  the 
size  of  Ishtar  Terra,  in  relation 
to  Aphrodite  Terra,  is  exagger- 
ated. This  is  similar  to  the  exag- 
geration of  North  America's  size 
compared  with  Africa  or  India 
on  Mercator  projections  of 
Earth 's  surface. 

circular  features  that  looked  much  like  young 
impact  craters.  Ejected  material  had  produced 
a  surrounding  rough  area,  bright  on  the  radar 
images.  The  existence  of  these  young  craters 
was  also  confirmed  by  Magellan  images. 

Orbiter  discovered  that  Venus'  lowlands  cover 
about  25%  of  the  surface.  By  contrast,  terrestrial 
lowlands  cover  70%  of  Earth.  Plateaus  and  moun- 
tains on  Venus  are  as  high  as  or  higher  than 
those  on  Earth.  The  lowlands,  however,  are  only 
one-fifth  the  greatest  depth  of  Earth's  lowlands. 

A  vast  lowland  basin,  Atalanta  Planitia,  is  cen- 
tered at  170°  east  longitude  and  65°  north 
latitude.  It  is  about  the  size  of  Earth's  North 
Atlantic  Ocean  basin.  The  smooth  surface  of 
Atalanta  Planitia,  about  2  km  (124  miles) 
below  the  mean  elevation,  resembles  the  mare 
basins  of  the  Moon.  Because  there  were  few 
circular  bright  features  that  could  be  impact 
craters  on  these  lowland  areas,  scientists 
thought  that  the  surface  may  be  young.  The 

basin  forms  part  of  a  large  belt  of  irregular, 
unconnected  lowlands  encircling  the  planet, 
which  were  later  discovered  to  be  lava-flooded 

Precise  observations  of  Pioneer's  orbit  around 
Venus  allowed  researchers  to  map  the  gravity 
field  in  detail.  One  theory  for  these  gravity 
anomalies  was  that  the  plains  have  a  thin  crust 
of  lower  density  than  that  below  the  upland 
plains.  This  is  similar  to  conditions  on  the  Moon 
and  Mars.  Some  geologists  also  suggested  that 
the  low  areas  are  depressions  that  later  filled 
with  basaltic  lavas.  This  is  similar  to  the  mare 
surfaces  of  the  Moon  and  some  of  the  plains 
on  Mars.  Others  theorized  that  they  could  be 
filled  with  now  consolidated  windblown  sedi- 
ments. Magellan  images  later  showed  that  lava 
flows  have  globally  modified  Venus'  surface. 

There  are  only  two  highland  or  continental 
masses  on  Venus:  Ishtar  Terra  and  Aphrodite 
Terra  (Figure  6-5).  A  much  smaller  elevated 




Vertical  exaggeration  200X 

L  Maxwell  Montes 



Figure  6-6.  Ishtar  Terra,  the 
northern  continental  mass. 
(Top)  Artist's  concept  of 
Ishtar  Terra  compared  with 
an  outline  of  the  United 
States  to  show  the  relative 
sizes  of  the  two  land  masses. 
(Bottom)  A  cross  section  of 
Ishtar  Terra  shows  the  rela- 
tive heights  of  the  mountains, 
the  central  plain,  and  the 
surrounding  territory.  Note 
that  the  vertical  scale  is 
exaggerated  by  200  times 
the  horizontal  scale. 

region,  Beta  Regio,  appeared  to  be  a  volcanic 
area  connected  with  a  major  rift  valley  system. 
It  is  now  known  that  Beta  Regio  is  a  young 
region,  and  its  volcanic  mountains  may  still  be 
forming.  Ishtar  Terra  may  be  somewhat  older, 
while  the  oldest  region  may  be  Aphrodite  Terra. 
Atla  Regio,  the  "Scorpion's  Tail"  at  the  east 
end  of  Aphrodite,  also  may  be  young. 

Points  on  Ishtar  rise  to  about  11  km  (6.8  miles) 
and  on  Aphrodite  to  about  5  km  (3  miles) 
above  the  mean  radius  of  the  planet.  Only  5% 
or  6%  of  the  surface  in  these  "continental" 
regions  is  more  than  1600  m  (5200  ft)  above 
the  mean  level.  This  measurement  compares 
with  30%  on  terrestrial  continents.  The  mass 
of  these  regions  is  about  80%  compensated. 
Three  possible  causes  explain  this  compensa- 
tion: mantle  convection  is  underplating  the 

highland  masses  with  silicic  rocks,  mantle 
plumes  of  upwelling  magma  are  producing 
local  differentiation  to  balance  the  thickness  of 
the  crust,  or  plate  tectonics  are  causing  conti- 
nental growth.  Continental  growth  by  tectonics 
does  not  have  supporting  evidence  of  deep 
subduction  troughs  or  midbasin  ridges.  These 
features  are  typical  of  terrestrial  plate  tectonics. 
The  presence  of  some  complex  forms  of  troughs 
and  ridges  in  many  areas  suggested  large-scale 
motions  of  the  crust,  but  terrestrial-style  plate 
tectonics  is  unlikely  on  Venus. 

Ishtar  Terra,  about  the  size  of  Australia  or  the 
continental  United  States  (Figure  6-6),  has  the 
highest  peaks  on  Venus.  There  are  three  geo- 
graphic units:  Maxwell  Montes,  Lakshmi  Planum 
(with  mountain  ranges  of  Akna  Montes  and 
Freyja  Montes  on  its  northern  and  northwestern 

margins),  and  an  extension  of  the  Lakshmi 
Planum.  Lakshmi  is  about  4  to  5  km  (2.5  to 
3.1  miles)  above  the  mean  level  of  Venus.  This 
is  about  the  same  general  elevation  as  the 
terrestrial  Tibetan  plateau  is  above  Earth's  mean 
sea  level.  It  is  twice  the  area  of  the  largest 
terrestrial  plateau.  Researchers  credited  a  bright 
scarp  on  the  southern  boundary  to  talus  slopes 
of  eroded  debris  along  a  fault  zone.  Such  a 
rough  surface  could  account  for  the  strong 
radar  reflection  that  observers  noted. 

If  Ishtar  consists  of  basaltic  lava  flows,  scientists 
expected  there  would  be  a  large  gravity  anomaly. 
But  the  data  from  Orbiter  showed  a  rather  mild 
positive  anomaly.  This  suggested  that  Lakshmi 
Planum  might  consist  of  thin  lavas  overlying 
an  uplifted  segment  of  ancient  crust.  This 
would  be  similar  to  the  Tharsis  region  of  Mars. 

On  the  eastern  side  of  Ishtar,  peaks  of  the 
towering  Maxwell  Montes  thrust  high  into 

Venus'  sky  (Figure  6-7).  Maxwell  was  first 
recognized  on  Earth-based  radar  images.  It  has 
a  great  circular  feature  that  may  be  a  caldera 
about  100  km  (62  miles)  across  and  1  km 
(0.62  mile)  deep.  The  caldera  is  offset  toward 
the  east  flank  of  the  mountain  some 
2  km  (1.24  miles)  below  the  summit.  No  bright 
flows  radiate  from  this  caldera.  The  assump- 
tion is  that  erosion  has  smoothed  any  lava 
flows.  If  so,  the  volcano  must  be  much  older 
than  those  in  Beta  Regio.  Many  slopes  on 
Maxwell  are,  however,  bright  in  the  radar 
images.  Such  brightness  suggests  that  they  are 
covered  with  rocks  that  scatter  the  radar  signal 
(probably  because  they  are  covered  with  debris). 
Alternatively,  they  could  be  bright  because  of 
extremely  steep  slopes.  Magellan  data  suggest 
one  is  a  6-km  (3.7-miles)  high  cliff. 

Scorpion-shaped  Aphrodite  Terra  (Figure  6-8) 
is  about  the  size  of  Africa.  It  has  two  mountain- 
ous areas.  On  the  east,  mountains  rise  5.7  km 

Figure  6-7.  An  imaginary 
view  of  Maxwell  Montes  as 
you  might  see  it  across  the 
plains  of  Venus.  This  is  based 
on  exploratory  data  from  the 
Pioneer  Venus  mission.  In  the 
late  1 980s,  new  computer 
techniques  enhanced  images 
from  exploratory  spacecraft. 
For  example,  such  techniques 
were  able  to  process  higher 
resolution  Magellan  space- 
craft images.  These  images 
allowed  analysts  to  move  a 
viewpoint  sequentially  over  a 
three-dimensional  image  of 
Venus'  surface.  This  gave  the 
impression  of  viewing  the 
mountains  and  valleys  from 
a  low-flying  aircraft.  At  pub- 
lication time,  such  programs 
for  Mars  and  Venus  are 
already  available  commer- 
cially for  personal  computer 
users  and  schools. 


Vertical  exaggeration  200X 
Aphrodite  Terra 



Figure  6-8.  Aphrodite  Terra, 
the  largest  continental-type 
region  of  Venus.  (Top) 
Artist's  concept  compares 
Aphrodite  Terra  with  the 
continental  United  States. 
(Bottom)  Section  across 
Aphrodite  to  show  the 
relative  heights  of  the 
main  features  compared 
with  the  surrounding  area. 
Again,  the  vertical  scale  is 
exaggerated  200  times 
the  horizontal. 

(3.5  miles)  above  the  mean  radius  of  Venus.  On 
the  west,  claw-shaped  mountains  are  about  4  km 
(2.5  miles)  high.  Between  them  are  rolling 
uplands.  Also,  there  is  a  topographically  com- 
plex mountain  rising  about  3  km  (1.9  miles) 
above  the  uplands.  The  mountains  have  very 
rough  surfaces  like  those  of  the  Ishtar  continent. 
South  of  Aphrodite  is  a  large  curved  feature 
called  Artemis  Chasma. 

Pioneer  images  of  Venus'  highland  areas  did 
not  show  circular  features  that  could  be  craters. 
With  radar,  craters  would  be  difficult  to  detect 
on  rough  terrain.  The  presence  of  these  high- 
lands may  confirm  lack  of  water  in  Venus' 
crust.  This  is  because  high  surface  temperatures 
would  readily  deform  water-rich  crustal  rock, 
so  highland  areas  could  not  persist. 

The  bright  radar  area  of  Beta  Regio  is  also  an 
interesting  region  dominated  by  a  large  com- 
plex shield  volcano  and  a  large  trough 
(Figure  6-9).  The  trough  is  part  of  a  fault  zone 
that  may  extend  far  to  the  south  where  two 
small  highland  areas  (Phoebe  Regio  and 
Themis  Regio)  are  aligned.  Other  small  high- 
lands, including  Asteria  Regio,  located  west  of 
Beta  Regio,  have  a  north-south  trend.  Lava 
flows  extend  radially  from  the  volcanic  centers. 
Two  Soviet  spacecraft  landed  directly  east  of 
Beta.  They  measured  gamma-ray  emanations 
from  the  surface  that  indicate  the  presence  of 
"basalts."  The  highest  mountainous  features 
on  Beta  Regio,  Theia  Mons,  and  Rhea  Mons,  are 
4  km  (2.5  miles)  high.  They,  too,  have  volca- 
noes. A  large  southward  trending  ridge  has 
elevations  up  to  2  km  (1.24  miles).  The  images 

showed  a  flat  terrain  west  of  Beta  Regio.  On  it 
was  a  linear  tectonic  feature  extending  4500  km 
(2796  miles)  to  the  south-southwest. 

Geologists  found  the  new  information  about 
this  region  very  interesting.  At  first,  from 
Earth-based  radar  data,  Beta  seemed  to  be  a 
shield  volcano  with  a  central  caldera.  Data 
from  Pioneer  Venus  suggested  that  it  is  part  of 
an  upland  area  of  volcanics.  A  great  rift  valley 
splits  this  region  of  Beta  Regio.  The  valley  has 
high  shoulders.  Its  nearest  Earthly  analogue  is 
the  Great  African  rift  valley  system.  Bright 
radial  streaks  radiating  from  the  shield  volca- 
noes are  suggestive  of  lava  flows.  Scientists 
suggested  that  their  presence  showed  Beta 
Regio  is  a  young  geologic  feature. 

Alpha  Regio  is  a  plateau  within  the  rolling  plains. 
It  is  located  at  25°  south  latitude  and  5°  east 
longitude  (that  is,  near  the  origin  of  longitude 
coordinates  on  Venus).  One  of  the  brightest 
features  on  Venus,  Alpha  Regio  is  elevated 
about  0.4  km  (1600  ft)  above  the  mean  level. 
Its  rim  is  2  km  (1.24  miles)  high.  Many  frac- 
tures cut  its  surface. 

Orbiter's  radar  revealed  many  rift  valleys  on 
Venus  (Figure  6-10).  They  appear  to  be  straight, 
or  gently  curved,  tectonic  features.  Some  are 
5000  km  (3000  miles)  long,  and  in  some 
regions  they  form  striking  patterns.  There  are 
many  valleys  east  of  Aphrodite  and  east  of 
Ishtar.  Geologists  suggest  regional  tectonic 
distortions  probably  caused  them. 

Figure  6-9.  The  great  volcanic 
area  of  Beta  Regio  appears  in 
this  artist's  drawing.  It  is  based 
on  data  from  Pioneer  Orbiter's 
radar.  The  area  has  many 
calderas,  and  scientists  believe  it 
is  one  of  the  youngest  areas  of 
the  planet,  with  the  most  recent 
volcanic  activity. 


ftgtvre  6-  7  0.  /4rt/st's  concept  of 
one  of  many  rift  valleys  that 
Pioneer  Orbiter  discovered  on 
Venus.  Often  the  valleys  have 
lines  of  mountains  associated 
with  them. 


The  gravity  field  of  Venus  mapped  by  the 
Orbiter  closely  matches  the  topography.  East 
of  Ishtar,  a  large  region  extends  from  14°  to 
40°  longitude  and  from  50°  to  75°  north  lati- 
tude. It  consists  of  complex  ridges  and  troughs, 
probably  disrupted  by  extensive  faulting.  This 
appeared  to  be  Venus'  most  tectonically 
disturbed  region.  Geophysicists  theorized  that 
this  region  could  be  where  plate  tectonics 
started  or  where  a  plume  of  hot  magma  rose 
through  the  mantle  to  produce  a  thickened 
low  density  crust.  Other  features  on  the  radar 
images  also  suggested  tectonic  activity  on 
Venus:  vertical  uplift  at  Lakshmi  Planum,  and 
the  northern  and  western  mountainous  ridges 
marginal  to  Ishtar  Terra.  These  ridges  may  be 
due  to  plate  motion.  However,  scientists  saw 
no  evidence  for  integrated  plate  tectonics  on 
Venus.  Development  of  thin  crusted  lowlands 
and  thick  crusted  highlands  would  imply  a 
long  period  of  widespread  mantle  convection 
early  in  Venus'  history.  Within  the  limits 
imposed  by  the  resolution  of  Earth-based  and 

Orbiter  radars,  scientists  concluded  that,  if 
plate  tectonics  took  place  on  Venus,  they  are 
grossly  different  in  character  from  terrestrial 
plate  tectonics.  The  more  detailed  Magellan 
radar  images  confirmed  these  conclusions. 

Venus  appears  different  from  the  other  Earth- 
like  planets.  There  are  signs  of  regional 
placements,  which  may  be  evidence  of 
incipient,  rudimentary,  or  past  plate  tectonics. 
Development  of  plate  tectonics  may  have 
stopped  because  Venus  lacks  water,  but  there  is 
no  proof  that  the  presence  of  much  water  has 
anything  to  do  with  plate  tectonics.  Geophysi- 
cists speculated  on  why  Venus  should  be  so 
different  from  Earth  when  it  is  so  similar  in 
many  respects.  They  suggested  that  the  higher 
surface  temperatures  led  to  domination  of 
tectonics  by  a  thick  layer  of  basaltic  material. 
This  could  not  be  subducted.  The  global  lava 
flows,  which  Magellan  revealed,  appear  to 
confirm  this.  The  global  distribution  of  impact 
craters,  with  little  evidence  of  a  widespread 

Figure  6-11.  The  USCS  processed 
the  radar  data  from  Pioneer  into 
perspective  views  of  the  surface 
showing  surface  relief  in  graphic 
detail.  This  computer  enhanced 
image  shows  Aphrodite  Terra 
from  the  southeast. 

ancient  crust  like  that  on  Mars  and  the  Moon, 
also  supports  their  suggestion. 

Some  computer  enhanced  surface  relief  images 
in  Figures  6-11  through  6-15  are  representa- 
tive of  the  first  spacecraft  radar  data  on  Venus. 
The  United  States  Geological  Survey  (USGS) 
processed  these  Pioneer  Venus  data  to  create 
the  first  three-dimensional  images  of  the 
planet's  surface.  These  clearly  show  great 
depressions  and  mountains.  The  areas  shown 
are  Aphrodite  Terra  (Figure  6-11),  Ishtar  Terra 
looking  toward  the  east  (Figure  6-12),  and  Beta 
Regio  (Figure  6-13). 

Two  Mercator  projections  based  on  the  Pioneer 
Venus  radar  data  provide  the  first  detailed  con- 
tour map  of  Venus'  surface.  Figure  6-14  is  an 
annotated  map  showing  the  major  topographi- 
cal features  discovered  by  the  Pioneer  Venus 
mission.  On  the  map,  the  chart  to  the  right 
shows  the  color  scale  indicating  height  in  terms 
of  the  planet's  radius,  together  with  a  kilometer 
scale  above  and  below  the  mean  radius. 

The  final  image  in  this  group  (Figure  6-15) 
shows  a  detailed  strip  of  the  equatorial  region 
of  the  planet  in  terms  of  radar  brightness  (see 
the  scale  on  the  right). 


Figure  6-12.  Ishtar  Terra  in  this 
computer  enhanced  relief  image 
is  from  the  west  looking  across 
Akna  Mantes  and  Lakshmi 
Planum  toward  Maxwell  Monies 
(the  red  area  just  above  center). 

Figure  6- 1 3.  Beta  Regio  is  the 
yellow  area  at  top  right  of  this 
relief  image.  Phoebe  Regio  is 
slightly  below  and  to  the  left. 
The  view  is  from  the  southeast. 


The  Atmosphere 

Pioneer  Venus  Orbiter  greatly  extended  obser- 
vations of  ultraviolet  patterns  in  Venus'  clouds. 
While  Mariner  10  obtained  eight  days  of  pic- 
tures, Pioneer  Venus  obtained  many  hundreds 
of  days  of  pictures.  These  images  provide  a 
greatly  improved  record  of  the  bulk  motions 
of  the  cloud  tops.  As  Venus  moved  around  the 
Sun  once  every  225  days,  the  cloud  photopola- 
rimeter  was  able  to  view  Venus  at  all  phases.  It 
imaged  the  planet  from  waxing  crescent  phase 
to  full  phase  and  back  to  waning  crescent  phase. 

Figure  6-16  is  the  first  image  that  Pioneer  Venus 
obtained.  The  low  contrast  is  due  to  the  oblique 
viewing  conditions  at  crescent  phase  combined 
with  high  altitude  haze  in  the  atmosphere. 

Figure  6-17  was  obtained  at  the  time  the  Soviet 
entry  probe  Venera  1 1  arrived.  Venera  descended 
at  the  equator  near  the  bright  limb  (left  edge) 
of  this  image. 

Figure  6-18  shows  Venus  at  full  phase.  Both 
poles  have  bright  caps.  An  optically  thick  haze 
of  small  particles  (radius  about  0.5  micron) 
above  the  main  cloud  layer  caused  these  caps. 

The  cloud  photopolarimeter  captured  the  data 
for  Figure  6-19  on  February  19,  1979,  when  the 

Sun  illuminated  almost  the  entire  hemisphere 
visible  from  the  spacecraft.  Large-scale  cloud 
patterns  show  a  horizontal  Y-pattern  previ- 
ously identified  by  lower  resolution  ultraviolet 
telescopic  sightings  from  Earth.  The  mottled 
small-scale  features  in  the  center  and  left  of 
center  in  the  image  probably  represent 
convection  cells  driven  by  the  Sun's  heat. 

A  question  arising  from  the  Mariner  10  data 
was  whether  the  features  that  move  around  in 
a  four-day  period  are  bulk  movement  of  atmo- 
spheric masses  or  wave  motions  in  the  atmo- 
sphere. The  Pioneer  probe  results  suggested 
that  the  air  is  moving  at  about  100  m/sec 
(330  ft/sec).  Probe  data  showed  that  the  wind 
velocity  starts  to  decrease  below  the  clouds.  At 
the  surface,  it  is  very  small.  Scientists  regard 
the  large  ultraviolet  features,  especially  the 
Y-  and  C-markings,  as  special  kinds  of  waves 
that  move  around  the  planet  at  the  same  speed 
as  the  air.  All  four  probes,  and  some  Soviet 
probes,  showed  the  same  westward  motion 
with  little  or  no  north-south  motion. 

The  cloud  photopolarimeter  experiment  on 
Orbiter  obtained  hundreds  of  images.  These 
were  four-color  polarization  maps  and  images 
of  Venus  in  ultraviolet  light.  Orbiter  obtained 
them  when  its  orbit  was  farthest  from  the 

Figure  6-14. 
Continued  pro- 
cessing of  radar 
data  produced 
this  map,  an  im- 
proved Mercator 
projection  (see 
the  text  for  more 
information).  The 
color  scale  shows 
the  height  of  the 
features,  with  the 
main  features 
identified  by 
name.  Note  the 
increased  detail 
compared  with 
the  first  radar  map 
in  Figure  6-1. 

planet  (apoapsis)— 40,000  to  64,000  km 
(24,856  to  39,770  miles)  away.  At  these  times, 
the  spacecraft  was  moving  slowly.  During  the 
opposite  portion  of  the  orbit  near  periapsis,  the 
spacecraft  passed  at  high  speed  through  Venus' 
tenuous  upper  atmosphere.  During  the  nominal 
mission,  it  approached  within  160  km  (99  miles) 
of  the  surface.  The  spacecraft  repeated  its  close 
approaches  in  1992  at  even  lower  altitudes 
before  final  entry.  These  were  times  when  other 
instruments  sampled  atmospheric  composition. 

North  polar  regions  of  Venus  were  unusually 
bright  in  the  images  during  the  nominal 

mission.  The  polarimetry  data  show  that  a  vast 
haze  of  submicron  particles  causes  the  bright 
polar  caps.  These  particles  are  about  0.25  micron 
in  radius.  High-altitude  haze  also  was  present 
at  lower  latitudes,  particularly  in  the  morning 
sky.  The  haze  extended  vertically  over  at  least 
25  km  (15.5  miles),  reaching  down  into  the 
main  visible  cloud  layer  where  it  coexisted  with 
the  larger  (about  1  micron  radius)  sulfuric  acid 
cloud  droplets.  The  refractive  index  of  the  haze 
particles  was  1.45  to  0.04,  which  suggested 
that  their  chemical  composition  could  be  the 
same  as  that  of  the  main  cloud  deck.  This  was 
shown  by  the  amount  of  haze  above  and 

Figure  6- 1 5.  The  advanced 
processing  also  produced 
very  detailed  maps  of  various 
regions  of  the  planet.  The 
example  in  this  figure  is  one 
of  Venus'  equatorial  regions. 
This  map  identifies  radar 
brightness  of  the  various 
features  given  by  the  color 
scale  on  the  right  of  the 


Figure  6- 1 6.  On  December  5, 
1 978,  Orbiter  obtained  this 
image  of  Venus  appearing  as  a 
crescent.  A  high  altitude  haze 
obscured  ultraviolet  features. 

Figure  6- 1 7.  On  Christmas  Day, 
1978,  the  Soviet  Venera  1 7 
entry  probe  descended  into  the 
atmosphere  of  Venus.  About 
the  same  time,  Pioneer  Venus 
Orbiter  obtained  this  image  of 
the  planet.  The  Soviet  probe 
plunged  into  the  atmosphere 
near  the  equator  dose  to  the 
bright  limb  (left)  of  the  image. 


within  the  main  cloud  deck  in  the  polar 
regions,  decreasing  by  more  than  one-half 
during  the  primary  mission.  Chemical  and 
aerosol  processes  are  at  work  on  time  scales  of 
several  months  and  longer. 

Researchers  used  the  images  to  study  atmo- 
spheric circulation  and  its  relationship  to 
regional  cloud  patterns.  The  four-day  rotation 

period  of  Venus'  atmosphere  was 
first  determined  from  the  reap- 
pearance at  four-day  intervals  of  a 
faint  horizontal  Y-shaped  feature 
in  ground-based  ultraviolet  images. 
Pioneer  Venus  images,  taken  at 
24-hour  intervals,  show  the 
planet's  rotation  in  detail.  As  the 
Y-feature  rotated  around  the 
planet,  it  confirmed  the  rotation 

Wind  speeds  near  the  cloud  tops 
were  determined  by  tracking  small 
cloud  features.  These  measure- 
ments revealed  nearly  constant 
high-speed  zonal  winds,  about 
100  m/sec  (330  ft/sec)  at  the 
equator.  The  winds  decreased 
toward  the  poles.  At  cloud-top 
level,  the  atmosphere  rotated 
almost  like  a  solid  body.  This 
zonal  circulation  differed  from 
that  observed  by  the  1974 
Mariner  10  flyby.  (Mariner  found 
strong  midlatitude  jet  streams.) 
The  planetary  scale  patterns  of  the 
clouds  changed  during  the 
Pioneer  Venus  primary  mission. 
For  example,  the  dark  horizontal 
Y-shaped  feature  disappeared  for 
periods  of  a  few  weeks. 

The  cloud  photopolarimeter 
functioned  perfectly  during 
Pioneer  Venus'  extended  mission.  Observations 
of  aerosol  evolution  and  atmospheric  circu- 
lation over  many  years  contributed  valuable 
knowledge  about  processes  that  are  important 
components  in  Earth's  climate  system. 

Continued  observations  from  orbit  showed 
that  the  wind  speed  of  320  km/hr  (199  mph) 
near  the  equator  corresponds  to  the  rotation 

period  between  four  and  five  days 
at  most  latitudes  (Figure  6-20).  At 
higher  latitudes,  however,  the 
period  decreases  to  three-and-a-half 
days,  and  under  polar  hazes  an 
infrared  dipole  rotates  in  three 
days.  Another  phenomenon 
showed  up  near  the  equator.  It  was 
a  wave  of  brightening  that  was, 
perhaps,  a  thickening  of  the  haze 
layers.  The  wave  passed  through 
the  clouds  and  circled  the  planet  in 
four  days.  However,  this  wave  was 
mysteriously  absent  in  1982  and 
1983.  The  figure  also  shows  that 
the  distinct  midlatitude  "jet 
stream"  obtained  from  1974 
Mariner  10  images  was  missing  in 
the  Pioneer  Venus  observations. 

Scientists  have  put  forward  a  theory 
for  Venus'  rapid  easterly  winds.  The 
theory  suggests  that  the  nature  of 
the  general  circulation  varies 
between  wind  profiles  observed  by 
Mariner  10  and  those  observed  by 
Pioneer  Venus.  Such  changes  might 
be  linked  to  long-term  variations  of 
clouds  and  aerosols,  such  as  the 
appearance  and  disappearance  of 
polar  caps.  Observations  during  the 
extended  Pioneer  Venus  mission 
provided  more  data,  and 
researchers  continued  to  try  to 
resolve  these  questions. 

One  day  after  the  spacecraft  took  the  image  in 
Figure  6-19,  high  zonal  winds  changed  the 
atmospheric  pattern.  They  carried  the  clouds 
forming  the  prominent  Y-feature  from  right  to 
left  by  about  90°  in  longitude,  leaving  only  the 
tail  of  the  Y  visible  (Figure  6-21).  The  hemi- 
sphere of  the  planet  opposite  the  Y 
(Figure  6-22)  revealed  a  pattern  of  linear 

Figure  6-18.  This  image  shows 
the  planet  fully  illuminated.  It 
was  acquired  on  February  9, 

Figure  6- 1 9.  This  image  clearly 
shows  the  horizontal  Y  and 
bright  polar  hoods.  Orbiter 
obtained  this  image  on 
February  15,  1979. 


^^     .*# 




,".  Mariner  10 




I      • 
1       -I 


>N      '^ 

'  —              II                                                ^^ 



1            »*» 



*                                      *     i   * 



\    x*yV**u-«  .«/C>A  ' 


>      ^ 

^X       Xtajtfc*^/     ^^       I                       ^ 




Pioneer  Venus      \ 










<  i 

1          1          1          1          1 

1  A 


-90    -60    -30       0       30      60     90 


Figure  6-20.  Imaging  and 
polarimetry  by  Mariner  1 0  and 
Pioneer  Orbiter  show  how  wind 
velocities  at  various  latitudes  on 
Venus  changed  considerably 
between  the  two  missions 
(except  at  equatorial  regions). 

Figure  6-2 1 .  High  zonal  winds 
have  moved  the  top  of  the 
horizontal  Y-feature  toward  the 
left,  leaving  the  tail  a  prominent 
feature  in  this  image  from  Feb- 
ruary 1 6,  1 979.  Compare  this 
image  with  that  in  Figure  6- 1 9, 
which  shows  more  of  the 

Figure  6-22.  In  this  image 
(February  1 7,  1 979),  the  hemi- 
sphere of  the  planet  opposite 
the  Y-feature  has  a  pattern  of 
linear  features  nearly  parallel 
to  latitude  circles. 

features  nearly  parallel  to  the  latitude  circles. 
Curvilinear  features  predict  reappearance  of 
the  Y-feature,  which  was  recorded  in 
Figure  6-23. 


When  Pioneer  arrived  at  Venus,  both  poles 
were  covered  by  bright  cloud  caps,  which  had 
been  seen  on  one  or  both  poles  several  times 
during  earlier  Earth-based  sightings.  The  cloud 
photopolarimeter  experimenters 
identified  the  "cloud"  caps  as  a 
thick  blanket  of  small  haze 
particles,  about  0.5  micron  in 
radius.  A  series  of  parallel  dark 
bands  breaks  the  edge  of  the 
bright  polar  cap  (Figure  6-24). 
Bright  streamers  of  haze  particles 
extend  from  the  polar  cap 
toward  lower  latitudes  (Figure  6-25). 

Sunlight  becomes  polarized  when 
clouds  reflect  it.  The  nature  of  the 
polarization  can  provide  informa- 
tion about  the  physical  properties 
of  cloud  particles.  Studies  of 
ground-based  polarization  mea- 
surements of  Venus  had  already 
revealed  that  the  major  cloud  deck 
consists  of  spherical  sulfuric  acid 
droplets  1  micron  (1O4  cm)  in 

The  droplets  in  this  deck  produce 
positive  polarization  at  ultraviolet 
wavelengths.  This  pattern  appears 
at  the  center  of  the  disk  in  the 
Pioneer  Venus  polarization  map 
(Figure  6-26).  The  map  also  indi- 
cates anomalous  regions  of  nega- 
tive polarization  near  both  poles. 
Their  location  corresponds  to  the 
bright  polar  caps  in  Figure  6-27. 
This  image  was  obtained  just  five 
hours  before  the  polarization  map. 
Polarization  of  the  polar  caps 
indicates  a  thick  haze  of  very  small 
particles  (0.25  micron  in  radius) 
overlying  the  main  cloud  layer. 
Except  for  effects  of  their  small 

Figure  6-23.  On  this  image 
(February  18,  1979), 
curvilinear  features  predict  the 
reappearance  of  the  Y-feature. 

figure  6-24.  A  series  of 
parallel  dark  bands  breaks  the 
equatorial  edge  of  the  bright 
polar  cap. 

Figure  6-25.  (Above)  In  this 
image  (February  1 6,  7  979), 
bright  streamers  of  haze 
particles  extend  toward  lower 
latitudes  from  the  bright  polar 
cap  at  the  bottom  of  the  image. 


size,  the  polarization  properties  are  similar  to 
those  of  the  droplets  in  the  main  cloud.  This 
suggests  that  the  haze  also  may  be  composed 
of  sulfuric  acid. 

The  polar  haze  began  to  partially  disappear  in 
mid-1979.  The  number  of  haze  particles  above 
each  square  centimeter  of  the  main  cloud 

deck,  which  was  about  300  million  in  January 
and  February,  decreased  to  less  than  half  of 
that  over  a  period  of  several  months.  Contin- 
ued observations  during  the  extended  Pioneer 
Venus  mission  were  used  to  study  cloud  and 
haze  variations  and  their  possible  link  to  long- 
term  changes  of  atmospheric  dynamics. 

Figure  6-26.  (Top  left)  This 
drawing  shows  a  polarization 
map  that  Orbiter  obtained  on 
February  25,  1 979.  The  map 
shows  anomalous  regions  of 
negative  polarization  caused 
by  haze  near  the  poles.  It  also 
shows  positive  polarization  from 
sulfurk  add  cloud  drops  at  the 
center  of  the  disk. 

Figure  6-27.  (Top  right)  This 
image  was  obtained  several 
hours  before  the  polarization 
map  of  Figure  6-26.  The  rela- 
tion between  ultraviolet  features 
and  map  contours  is  clear. 


The  nature  of  these  aerosols  and  their  relation- 
ship with  climate  on  Venus  are  of  interest  for 
studies  of  Earth's  climate.  Similar  aerosols  are 
produced  in  Earth's  stratosphere  following  large 
volcanic  eruptions.  Some  scientists  believe  that 
they  may  cause  significant  climate  changes. 

Venus'  cloudy  atmosphere  reveals  a  rich  spec- 
trum of  dynamic  events,  especially  in  the 
equatorial  region.  Some  of  these  features  are: 

(a)  Bright-rimmed  cells  appear  as  mottled 
cellular  cloud  patterns.  Scientists  believe  these 
are  convection  cells  driven  by  the  Sun's  heat. 
They  may  have  some  analogy  to  tropical 
cumulus  cloud  clusters  on  Earth. 

(b)  Wave-trains  are  series  of  short  streaks  cut- 
ting across  background  features.  Their  almost 
vertical  lines  are  strongly  suggestive  of  a  wave 

(c)  Circumequatorial  belts,  vaguely  visible  in 
some  images,  appear  as  bright  lines  parallel  to 
the  equator,  where  they  stretch  several 
thousand  miles  from  the  limb  across  the  disk. 

The  atmosphere  was  probed  by  many  instru- 
ments from  the  Orbiter  and  sampled  by  others 

on  the  four  probes  and  the  Bus.  Regions  of  the 
atmosphere  are  generally  based  on  temperature, 
as  shown  in  Figure  6-28.  Solid  lines  represent 
data  collected  by  the  Probe  Bus  and  the 
Orbiter.  Dashed  lines  indicate  limited  data 
from  the  Small  Probes.  Direct  probe  measure- 
ments cover  the  range  from  the  mesosphere  to 
the  surface.  The  Orbiter  infrared  radiometer 
provided  almost  global  information  for  the 
stratosphere.  More  results  came  from  the  radio 
occultation  experiment.  All  these  data,  from 
the  surface  to  the  ionosphere,  provide  an 
almost  complete  picture  of  the  temperature, 
pressure,  and  density  structure  of  Venus' 

An  exciting  discovery  was  the  enormous  range 
of  temperature  between  day  and  night  in  the 
upper  atmosphere.  Yet,  even  on  the  dayside  of 
Venus,  the  upper  atmosphere  temperature  is 
not  as  hot  as  Earth's  upper  atmosphere.  On 
Earth,  temperatures  are  700  to  1000  K  at 
sunspot  minimum.  Heat  comes  from  formation 
of  the  ionosphere  by  very  short  wavelength 
solar  ultraviolet  radiation.  Somehow,  Venus 
manages  to  keep  a  lower  temperature  than 
Earth's  upper  atmosphere  even  with  twice  the 
flux  of  incoming  solar  radiation. 

But  the  real  surprise  is  the  low  temperature  of 
the  upper  atmosphere  on  the  nightside.  This 
region  cannot  be  called  a  thermosphere  (hot 
sphere)  like  the  equivalent  region  in  Earth's 
atmosphere.  The  thermosphere  is  the  atmo- 
spheric region  where  the  incoming  solar 
photons  are  absorbed  and  solar  heat  is  trans- 
ferred into  the  atmosphere.  Scientists  coined 
the  name  cryosphere  (cold  sphere)  to  describe 
this  cold  region  of  Venus'  upper  atmosphere. 
Although  the  Sun  does  not  directly  heat  the 
nightside,  heat  must  flow  to  the  nightside 
from  the  dayside.  It  also  must  flow  upward  on 
the  nightside  from  the  warmer  mesosphere. 
The  gradient  between  day  and  night  is  rather 
sharp,  occupying  little  more  than  the  twilight 
zones,  20°  to  30°  of  longitude.  Theories 
developed  to  describe  the  behavior  of  Earth's 
thermosphere  do  not  apply  to  Venus  and  leave 
unexplained  many  temperature  features  of 
Venus'  atmosphere.  Clearly,  improvements  in 
the  theory  were  needed.  Data  from  Phases  II 
and  III  made  it  possible  to  refine  models  and 
test  the  theories  against  these  new  data. 

Scientists  knew  that  the  dayside  thermosphere 
responds  to  short-term  changes  in  solar  activity, 
such  as  those  caused  by  the  Sun's  27-day 
rotation.  They  expected  that  it  would  also 
respond  to  changes  in  solar  activity  over  the 
11 -year  solar  activity  cycle.  Despite  solar  heat- 
ing, the  temperature  of  the  dayside  thermo- 
sphere is  only  about  300  K.  This  was  much 
colder  than  predicted.  Researchers  explained 
this  low  temperature  on  the  basis  of  eddy  and 
radiative  cooling.  Eddy  cooling  occurs  when 
heat  is  transported  down  into  the  mesosphere. 
Radiative  cooling  is  due  to  atomic  oxygen 
exciting  carbon  dioxide  into  a  strong  emission 
at  15  microns.  This  radiative  cooling  appears 
to  be  the  main  mechanism  keeping  the  thermo- 
sphere temperature  down.  On  this  basis,  some 
researchers  concluded  that  the  response  of  the 
thermosphere  to  the  11 -year  solar  activity 


250  500 

Temperature,  K 

Figure  6-28.  Typical 
temperatures  for  Venus ' 
atmosphere  relate  to 
altitude  of  the  various 
regions.  Corresponding 
heights  for  Earth's 
atmosphere  appear  for 

cycle  would  be  small.  Also,  they  predicted  that 
the  atomic  oxygen/carbon  dioxide  ratio  would 
increase  as  increased  solar  radiation  photodis- 
sociated  more  carbon  dioxide.  In  turn,  this 
would  increase  the  cooling  mechanism  and 
help  to  weaken  the  effects  of  solar  activity  on 
the  thermosphere's  temperature. 

New  data  provided  important  information 
about  the  atmosphere  between  130  and  210  km 
(80  and  130  miles)  on  both  nightside  and 
dayside  and  at  low  solar  activity.  These  were 
atmospheric  drag  measurements  from  the 
orbital  decay  of  the  Orbiter  in  1992,  coupled 
with  drag  data  from  the  Magellan  spacecraft. 
Researchers  compared  these  data  with  earlier 
data  they  obtained  at  high  solar  activity  in 
1978-1980.  The  result  has  been  an  increased 
knowledge  of  the  detailed  response  to  solar 
variations  of  temperature,  atomic  oxygen,  and 
carbon  dioxide  in  Venus'  thermosphere.  For 
example,  studies  of  images  in  light  from 
carbon  monoxide  and  oxygen,  made  by  the 
ultraviolet  spectrometer,  showed  that,  as  solar 



activity  declined,  the  gases  decreased  relative 
to  carbon  dioxide  (from  which  they  are 
derived  by  photodissociation).  The  effects 
reversed  after  solar  minimum. 

Investigators  found  a  weak  but  detectable 
temperature  response  on  the  dayside,  which 
was  in  accord  with  the  predicted  response 
based  on  strong  carbon  dioxide  radiative 
cooling.  This  was  an  important  discovery 
because  it  highlighted  a  mechanism  that 
might  cause  an  otherwise  unexpected  strong 
cooling  of  Earth's  thermosphere  if  terrestrial 
carbon  dioxide  builds  up  here  in  the  future. 

It  now  seems  clear  that  with  decreasing  solar 
activity  the  oxygen/carbon  dioxide  ratio  in  the 
lower  thermosphere  decreases,  as  indicated  by 
decreased  photodissociation  of  carbon  dioxide 
and  a  lower  temperature.  The  decrease  in  this 
ratio  results  in  less  effective  oxygen-carbon 
dioxide  cooling  and  a  partial  cancellation  of 
the  decreased  extreme  ultraviolet  heating  at 
times  of  low  solar  activity. 

Investigators  found  that  the  percentage 
decrease  in  atomic  oxygen  with  decreasing 
solar  activity  on  the  dayside  was  about  the 
same  as  that  of  atomic  oxygen  transported  to 
the  nightside.  Also,  they  saw  a  weak  response 
of  temperature  on  the  nightside  to 
solar  variations. 

Scientists  now  conclude  that  there  is  evidence 
of  photochemical,  radiative,  and  dynamical 
responses  of  Venus'  upper  atmosphere  to 
changes  in  solar  activity.  On  the  dayside,  there 
is  a  weak  temperature  response  to  long-term 
solar  activity  variations.  Also,  carbon  dioxide 
increases  in  the  lower  thermosphere  with 
decreasing  solar  activity.  This  is  due  to  reduced 
photodissociation  of  carbon  dioxide.  The  con- 
sequence is  a  strong  decrease  in  the  ratio  of 
atomic  oxygen  to  carbon  dioxide  in  the  lower 

thermosphere  when  solar  activity  is  low.  In 
turn,  this  leads  to  a  reduction  in  the  amount 
of  cooling  by  the  atomic  oxygen-carbon 
dioxide  mechanism.  Scientists  consider  that 
similar  effects  might  operate  in  the  Earth's 
upper  atmosphere,  and  that  further  study 
is  needed. 

On  Venus'  nightside,  researchers  found  an 
unexpected  strong  response  of  atomic  oxygen 
to  changes  in  solar  activity.  The  atomic 
oxygen  on  the  nightside  reflects  the  changes 
in  atomic  oxygen  on  the  dayside.  Also,  there  is 
a  decrease  in  the  day-to-night  flux  across  the 
terminator  at  low  solar  activity. 

Because  the  nightside  is  so  cold,  atmospheric 
pressure  falls  very  rapidly  with  increasing 
height.  At  each  atmospheric  level,  pressure  is 
much  less  than  on  the  dayside.  The  nightside 
high  atmosphere  is  exceptionally  cold.  Its 
temperature  of  100  K  is  less  than  any  other 
planetary  atmosphere  closer  to  the  Sun  than 
Saturn.  This  large  difference  between  day  and 
night  temperature  causes  very  strong  winds  to 
blow  from  day  to  night.  The  Orbiter  observed 
this  large  temperature  difference  directly. 
Unfortunately,  it  did  not  carry  an  instrument 
to  observe  the  winds  directly.  However,  there 
were  indirect  confirmations  of  their  presence. 

Data  gathered  by  Orbiter's  instruments  also 
showed  that  the  high  atmosphere,  like  the 
cloud  top  regions,  "superrotates"  much  faster 
than  the  planet  itself.  Violent  high-altitude 
winds  are  patterned  by  density  waves  that 
begin  in  the  lower  atmosphere.  They  appear  in 
the  data  from  the  neutral  mass  spectrometer 
and  as  bright  streaks  in  the  images  from  the 
ultraviolet  spectrometer. 

The  bottom  65  km  (40  miles)  of  Venus'  atmo- 
sphere is  the  troposphere.  The  boundary  of  this 
region,  the  tropopause,  coincides  with  the  cloud 



>>  106  - 



-i  300 

-   200 





-   100 


6  p.m. 


6  a.m. 


f/gttvre  6-29.  Instruments 
onboard  Orbiter  revealed  a 
very  cold  nightside  thermo- 
sphere,  or  cryosphere.  Resulting 
pressure  gradients  drive  strong 
winds  from  the  dayside  to  the 
nightside  of  the  planet.  Atomic 
oxygen  densities  show  a  strong 
cold-trapping  effect  on  the 
nightside  similar  to  the  CO2 
curve  in  the  diagram.  The 
displacement  of  the  nightside 
atomic  hydrogen  peak  toward 
the  dawn  terminator,  as 
indicated  by  the  H  curve, 
suggests  that  the  thermo- 
spheric  winds  have  a  super- 
rotating  component. 

tops.  Pressures  at  the  tropopauses  of  Earth  and 
Venus  are  similar,  but  their  heights  are  quite 
different  because  of  different  surface  pressures. 

dioxide.  However,  very  scarce  atoms,  such  as 
chlorine,  probably  reduce  oxygen  and  ozone 
to  levels  that  make  their  detection  impossible. 

Above  the  tropopause  is  the  middle  atmo- 
sphere. On  Earth,  this  region  consists  of  the 
stratosphere  and  mesosphere.  The  boundary 
between  them  is  a  temperature  maximum 
caused  by  ozone  absorbing  solar  ultraviolet 
radiation.  Venus  has  no  detectable  ozone  and 
no  temperature  maximum  to  divide  its  middle 
atmosphere.  Scientists  have  not  yet  agreed 
upon  a  single  name  for  this  combined  region 
in  Venus'  atmosphere.  They  believe  the  middle 
atmosphere  has  much  chemical  activity  driven 
by  solar  ultraviolet  radiation.  Oxygen  and 
ozone  must  continue  to  be  released  into  the 
atmosphere  by  the  breakdown  of  carbon 

Data  from  the  neutral  mass  spectrometer  and 
the  atmospheric  drag  experiment  confirmed 
a  very  cold  nightside  thermosphere,  or 
cryosphere.  The  resulting  pressure  gradients 
drive  strong  winds  from  the  planet's  dayside 
to  its  nightside.  Researchers  deduced  atomic 
hydrogen  densities  from  ion  mass  spectrom- 
eter data  on  H+  and  O+.  They  obtained  densi- 
ties of  oxygen  from  neutral  mass  spectrometer 
data.  The  hydrogen  peaked  on  the  nightside, 
and  the  bulge  was  displaced  toward  the  dawn 
terminator  (Figure  6-29).  The  oxygen  densities 
showed  that  there  was  a  strong  cold-trapping 
effect  on  the  nightside.  The  displacement  of 



the  hydrogen  peak  showed  that  thermospheric 
winds  have  a  superrotating  component. 

Thermal  contrasts  provide  the  driving  mecha- 
nism for  general  atmospheric  circulation.  They 
set  up  pressure  differences  to  drive  the  flow. 
Pioneer  made  a  major  discovery  about  the 
lower  atmosphere  below  the  clouds.  There  is 
little  thermal  contrast  between  night  and  day 
and  from  the  equator  to  60°  latitude.  Thus, 
temperature  variations  at  and  near  Venus' 
surface  are  small. 

Absence  of  large  thermal  contrast  in  the  atmo- 
sphere led  scientists  to  several  other  conclusions. 
At  some  levels,  there  must  be  an  effective  trans- 
port of  heat  from  equator  to  poles  and  from 
the  subsolar  to  the  antisolar  points  by  atmo- 
spheric circulation.  The  atmosphere  must  trans- 
port heat  efficiently  from  the  region  below  the 
Sun  to  the  rest  of  the  planet.  Because  the  lower 
atmosphere  is  so  dense,  slow  winds  alone  are 
sufficient.  For  the  same  reason,  the  rate  at 
which  temperature  rises  or  falls  due  to  varying 
inputs  of  solar  heat  is  small. 

A  surprising  discovery  was  that  most  of  the  deep 
atmosphere  is  stable  and  stratified  like  Earth's 
stratosphere.  An  analogy  is  the  stagnant  air  lay- 
ers in  the  Los  Angeles  basin  on  a  smoggy  day. 
From  the  clouds  down  to  30  km  (18.6  miles) 
altitude,  a  layer  23  km  (14  miles)  deep,  and  in 
a  lower  layer  between  15  and  20  km  (9  and 
12  miles)  altitude,  the  atmosphere  is  stratified 
and  free  of  convective  activity.  It  does  not  rise 
and  overturn  in  the  way  that  air  does  on  Earth 
over  hot  farm  or  desert  lands,  or  in  cumulus 
clouds.  This  was  unexpected  because  scientists 
thought  that  the  high  temperatures  in  the  deep 
atmosphere  would  be  a  source  of  hot,  rising 
gas.  If  true,  this  would  lead  to  deep  convective 
cells  and  turbulence.  Also,  before  Pioneer  Venus, 
theoretical  studies  indicated  that,  at  radiative 
equilibrium,  much  of  the  lower  atmosphere 

would  be  unstable  and  overturning.  The 
Pioneer  Venus  data  quickly  led  scientists  to 
revise  these  earlier  models. 

All  four  probes  and  several  Soviet  probes 
measured  high  surface  temperatures.  They 
were  equal  within  uncertainties  of  a  degree  or 
so  when  corrected  to  a  constant  distance  from 
the  center  of  Venus.  Earth-based  instruments 
also  have  sensed  surface  temperatures  at  radio 
wavelengths,  with  comparable  results.  One 
thing  that  sets  Venus  apart  from  Mars  and 
Earth  is  its  very  high  surface  temperature.  One 
main  objective  of  the  Multiprobe  mission  was 
to  test  the  belief  that  the  "runaway  greenhouse 
effect"  caused  the  high  surface  temperature. 
This  effect  requires  that  only  a  small  percent- 
age of  the  solar  energy  reaching  the  surface  be 
converted  into  heat,  and  be  redistributed 
globally.  Further,  the  atmosphere  and  clouds 
must  form  an  insulating  blanket  that  infrared 
radiation  can  penetrate  only  with  difficulty. 
The  heat  cannot  be  reradiated  into  space. 

Pioneer  Venus  data  left  no  doubt  that  a  strong 
greenhouse  mechanism  is  at  work.  This  mech- 
anism describes  the  state  of  the  atmosphere 
above  about  35  to  50  km  (22  to  31  miles) 
altitude.  Below  that,  dynamics  control  the 
temperature.  Radiative  heating  associated  with 
the  greenhouse  mechanism  drives  the  dynamics. 
About  half  the  heating  of  the  atmosphere  by 
incoming  solar  radiation  occurs  near  the  top  of 
the  clouds.  The  rest  of  the  energy  is  distributed 
at  lower  altitudes  and  at  the  surface. 

Measured  infrared  fluxes  on  the  probes  showed 
several  anomalies.  These  anomalies  suggest 
that  parts  of  the  atmosphere  transmit  upward 
about  twice  the  energy  available  from  solar 
radiation  at  the  same  level.  Instrument  errors 
in  this  difficult  measurement  may  be  respon- 
sible. A  possibility  is  that  two  of  the  probes 
entered  regions  that  are  unusually  transparent 

to  thermal  radiation.  However,  this  is  unlikely 
because  much  of  the  absorption  is  due  to 
carbon  dioxide.  Scientists  suggested  that  the 
heat  balance  oscillates  around  its  average  state. 
Also,  the  anomalous  measurements  occurred 
during  the  cooling  phase.  Despite  these 
problems  in  interpreting  some  observations, 
the  runaway  greenhouse  effect,  coupled  with 
global  dynamics,  is  accepted  as  explaining  the 
high  surface  temperature. 

The  Atmosphere — Clouds 

When  viewed  from  the  Earth  in  visible  light, 
the  disk  of  Venus  appears  to  be  completely 
covered  with  a  bright  veil  of  unchanging, 
featureless,  yellowish  clouds.  Before  Pioneer 
Venus,  astronomers  had  diligently  observed 
these  clouds  from  Earth  without  adding  much 
to  our  knowledge.  Some  in  situ  data  through 
the  cloud  depths  were  available  from  Soviet 
missions  to  Venus,  especially  from  Veneras  9 
and  10.  Earlier,  Mariners  5  and  10  flyby  space- 
craft experiments  also  yielded  some  informa- 
tion, primarily  about  regions  near  the  cloud 
tops.  An  objective  of  Pioneer  was  to  determine 
the  nature  and  composition  of  the  clouds. 

Earth-based  observations  first  revealed  the 
clouds'  featureless,  global  nature.  These 
sightings  were  at  both  visible  and  infrared 
wavelengths.  However,  astronomers  also  had 
discovered  features  at  near  ultraviolet  wave- 
lengths, hinting  at  some  form  of  horizontal 
cloud  structure.  Further,  these  features 
appeared  to  circulate  around  the  planet  about 
every  four  days.  Mariner  10  obtained  detailed 
imaging  of  Venus  to  confirm  this  four-day  rota- 
tion period.  It  also  obtained  detailed  measure- 
ments of  the  circulation  near  the  cloud  tops.  The 
images  showed  that  the  motions  are  generally 
zonal;  that  is,  parallel  to  Venus'  equator. 

Observations  from  Earth  also  provided  evi- 
dence about  the  detailed  properties  of  the 

particles  composing  the  uppermost  clouds. 
Scientists  measured  scattered  sunlight  that  had 
interacted  with  the  uppermost  layers.  Particu- 
larly useful  were  measurements  of  how  scat- 
tered sunlight  polarized  at  various  angles  of 
observation  of  the  clouds  relative  to  the  solar 
illumination.  Researchers  compared  these 
measurements  with  calculations  based  on 
models  that  considered  particles  with  various 
properties.  Best  agreement  was  found  when 
the  particles  were  all  assumed  to  be  spherical 
and  about  the  same  size.  Their  effective  radius 
was  about  1 .05  microns,  and  their  index  of 
refraction  was  1 .44  for  visible  light. 

These  conclusions,  coupled  with  spectroscopic 
data  obtained  from  Earth,  suggested  that  the 
upper  cloud  particles  were  principally  concen- 
trated sulfuric  acid. 

Optical  experiments  aboard  the  Veneras  9  and 
10  probes  as  they  fell  through  the  atmosphere 
obtained  data  consistent  with  these  conclu- 
sions. Analyses  of  data  from  light  scattering 
(nephelometer)  experiments  on  the  Soviet 
probes  showed  that  the  vertical  cloud  structure 
had  three  main  layers.  The  data  also  yielded 
information  about  variations  in  particle  sizes 
and  indices  of  refraction  in  each  of  these  layers 
and  the  regions  between  them. 

Data  from  these  experiments  also  suggested 
that  larger  particles  with  large  indices  of  refrac- 
tion were  present  at  lower  altitudes.  Scientists 
tentatively  identified  the  particles  as  large 
sulfur  droplets.  Furthermore,  since  sulfur 
seemed  a  likely  candidate,  they  also  proposed 
sulfur  crystals  as  a  high-altitude  absorber 
responsible  for  the  ultraviolet  contrasts. 

Although  invisible  from  Earth,  a  very  tenuous 
haze  was  revealed  on  the  Mariner  10  images. 
The  haze  layers  were  above  the  cloud  tops  at 
altitudes  of  70  to  80  km  (43  to  50  miles).  Also, 


Figure  6-30.  Comparison  of  results 
from  Pioneer  Venus  probes  and 
Venera  9  show  a  remarkable 
similarity  in  profiles  of  the  cloud 
layers  plotted  against  altitude. 
These  results  suggest  that  the 
cloud  system  is  global,  even 
though  similarities  in  features 
of  the  vertical  structure  do  vary 
because  of  what  may  be  changes 
in  large-scale  dynamics  of  the 
cloud  system. 




i  56 


|   52 

<  48 




Pioneer  Venus  Nephelometer  results 

Window  cover 

.  Lower  cloud 



I        I        I 


—        probe 

I        I 

—     probe 




Venera  9 




.5     1.0   1.5  2.0    0     .5     1.0    1.5     0      .5     1.0  1.5    0     .5     1.0    1.5 
175°  backscattering  cross  section  (m  Br  X  10"1) 

J | 

0  1.0       3.0       5.0 





bright  transitory  polar  caps  or  bands,  lasting 
from  weeks  to  months,  were  observed 
from  Earth. 

Based  on  the  above  background,  scientists  chose 
experiments  for  Pioneer  to  investigate,  in 
detail,  cloud  properties  at  depth  and  temporal 
"weather-related"  features  at  cloud  tops.  For 
example,  experiments  on  the  probes  were 
selected  to  detail  the  vertical  cloud  structure  at 
each  of  the  four  entry  sites.  Orbiter  experiments 
provided  many  years  of  cloud-top  observations. 

Primary  experiments  selected  specifically  to 
examine  clouds  included  equipment  such  as 
Large  Probe  and  Small  Probe  nephelometers, 
Large  Probe  cloud-particle  size  spectrometer, 
and  Orbiter  cloud  photopolarimeter/imager. 
Cloud-related  experiments  that  provided 
information  from  which  scientists  could  infer 
cloud  properties  included  several  pieces  of 
equipment.  These  were  the  Large  Probe  solar 
net  flux  radiometer,  the  Large  Probe  neutral 
mass  spectrometer,  the  Large  Probe  gas 
chromatograph,  Orbiter  infrared  radiometer, 
and  Orbiter  ultraviolet  spectrometer.  Further 
supporting  information  was  obtained  from  the 
Large  Probe  infrared  radiometer,  the  Small 
Probe  net  flux  radiometer,  and  the  Large  and 
Small  Probe  atmospheric  structure  experiments. 

Investigators  combined  data  from  in-depth 
measurements  from  the  four  probe  locations 
with  the  Orbiter's  planetwide  observations. 
Data  from  the  probes  served  as  "ground  truth" 
for  the  Orbiter's  data.  This  led  to  a  more 
complete  general  understanding  of  the  clouds, 
their  morphology,  the  microphysical  descrip- 
tion of  their  particles,  and  their  physical  and 
chemical  composition.  It  also  led  to  increased 
understanding  of  their  optical  properties,  their 
role  in  planetary  energy  processes,  and  their 
interaction  with  atmospheric  motions. 

Cloud  Morphology 

From  Pioneer  data,  scientists  identified  several 
particle-bearing  regions  in  Venus'  atmosphere: 

(a)  A  haze  region  extends  upward  from  70  to 
90  km  (43  to  56  miles).  It  is  composed  of  very 
small  particles  observed  by  the  Orbiter  cloud 
photopolarimeter,  ultraviolet  spectrometer, 
and  infrared  radiometer  experiments. 

(b)  The  main  cloud  deck  consists  of  three 
more-or-less  distinctly  separate  regions.  An 
upper  cloud  region  is  at  56.5  to  70  km  (35  to 
43  miles),  a  middle  cloud  region  at  50.5  to 
56.5  km  (31.4  to  35  miles),  and  a  lower  cloud 
region  at  47.5  to  50.5  km  (29.5  to  31.4  miles). 
Each  has  varying  microphysical  properties 
observed  by  the  probe  nephelometer  and 
cloud-particle  size  spectrometer  experiments. 

Table  6-1 .   Summary  of  Characteristics  of  Venus  Clouds 







Upper  haze 


-83  to  -48 


Sulfuric  acid 


+  contaminants 

Upper  cloud 


-48  to  1  3 


Sulfuric  acid 

0.4,  2.0 

+  contaminants 


Middle  cloud 


1  3  to  72 


Sulfuric  acid 

0.3,  2.5,  7.0 


+  contaminants 


+  crystals 

Lower  cloud 


72  to  94 


Sulfuric  acid 

0.4,  2.0,  8.0 

+  contaminants 


+  crystals 



94  to  1  05 


Sulfuric  acid 

0.3,  2.0 


+  contaminants 


Lower  haze 


94  to  209 


Sulfuric  acid 


+  contaminants 

(c)  A  lower  haze  extends  from  47.5  km  to 
about  31  km  (29.5  to  19  miles).  It  was 
observed  by  the  probe  cloud-particle  size 
spectrometer.  Also,  investigators  saw  evidence 
of  matter  suspended  in  the  atmosphere  at  lower 
altitudes.  Some  of  the  probes'  nephelometers 
provided  this  evidence. 

(d)  Additional  thin-layered  structures,  or  pre- 
cloud  layers,  exist  as  transitory  clouds  in  the 
upper  part  of  the  lower  haze  region. 

Figure  6-30  shows  the  results  of  the  nephe- 
lometer  measurements  of  the  clouds'  vertical 
structure  at  four  Pioneer  Venus  sites  and  one 
Venera  site.  Table  6-1  summarizes  properties 
of  the  hazes  and  main  clouds  assembled  from 
Pioneer  Venus  experiments. 

The  upper  and  lower  cloud  regions  are  much 
more  variable  in  structure  than  the  middle 
cloud  region.  By  analogy  with  Earth  clouds,  all 
clouds  are  stratiform.  They  consist  of  fairly 
large  scale,  uniformly  layered  structures.  With 
the  possible  exception  of  the  middle  cloud 
region,  the  cloud  regions  are  remarkably  stable 
against  vertical  overturning.  For  such  cloud 
structures,  there  may  be  a  possibility  of  light 
mist  or  drizzle.  Heavy  precipitation  typical  of 
cumulus-scale  convection  in  an  unstable 

atmosphere  is  unlikely.  Futhermore,  similari- 
ties in  the  main  cloud  deck  profiles  and  in 
stability  properties  (measures  of  the  atmo- 
sphere's tendency  to  overturn  by  convection) 
at  each  of  their  four  probe  sites  strongly 
suggest  that  the  major  features  of  the  cloud 
systems  are  global.  They  are  not  very  depen- 
dent on  local  longitude  or  latitude,  except 
perhaps  at  high  latitudes  and  at  the  equator. 

Scientists  suggest  that  the  features  observed  at 
ultraviolet  wavelengths  are  principally  identi- 
fied with  the  motion  of  an  ultraviolet  absorber 
in  the  atmosphere.  This  is  because  changes  in 
the  concentration  of  sulfuric  acid  particles  can- 
not account  for  these  patterns.  Also,  the  haze 
is  not  dense  enough  to  provide  the  observed 
contrasts.  The  ultraviolet  absorbing  species, 
other  than  sulfur  dioxide,  which  investigators 
have  identified  as  one  of  the  absorbing  species, 
remains  unknown.  Since  solar  energy  is 
absorbed  mainly  in  and  above  the  upper  levels 
of  the  main  cloud  deck,  vertical  motions  of 
this  unknown  species  from  below  the  haze 
may  be  responsible  for  the  observed  dark 
regions.  However,  such  an  assumption  would 
imply  a  dark  region  of  upwelling  ultraviolet 
radiation  absorber  at  the  subsolar  point  on  the 
planet.  Yet,  the  subsolar  point  is  a  bright  region. 


Polar  cap 

Dark  polar 

Bow  shape 




Bow  shape 

Dark  equatorial  band 

Bow  shape 


-  Bright  polar  band 



figure  6-31.  Basic  types  of  cloud 
features  observed  on  Venus  in 
ultraviolet  images.  These  views 
typically  occur  two  Earth-days 


Nonetheless,  researchers  think  that  the  absorber 
masks  motions  in  the  atmosphere  by  indicat- 
ing regions  of  horizontal  variation  or  of 
vertical  displacement  of  the  absorber.  This  is 
presumably  from  below  the  cloud  tops  to 
higher  altitudes  where  regions  of  ultraviolet 
absorption  would  appear  darker.  The  ultravio- 
let absorber  acts  as  a  marker  of  motion.  Also, 
since  it  absorbs  appreciable  amounts  of  energy, 
it  may  play  a  role  in  cloud  layer  dynamics. 

Ultraviolet  features  may  be  categorized  into 
those  associated  with  three  distinct  regions  of 
the  planet.  A  polar  zone  is  above  50°  latitude,  a 
midlatitude  zone  between  20°  and  50°,  and  an 
equatorial  zone  extends  about  20°  north  and 
south  of  the  equator.  A  small-particle  haze 
covers  the  planet,  varying  in  density  with 
latitude.  A  polar  haze  collar,  bright  in  ultravio- 
let light,  encircles  the  polar  regions  at  about 
55°  latitude.  However,  even  at  lower  latitudes, 
there  are  significant  amounts  of  haze  above 
the  cloud  tops.  Also,  there  is  evidence  of 
increased  amounts  of  haze  at  the  morning  and 
evening  terminators.  The  haze  even  covers  the 
polar  regions  where  it  obscures,  in  ultraviolet 
images,  features  that  can  be  seen  in  infrared 
images.  Changes  in  the  general  haze  features 

appear  to  occur  in  times  ranging  from  months 
to  years. 

The  large  variety  of  dark  features  in  the  ultra- 
violet images  of  midlatitudes  and  equatorial 
regions  is  composed  of  three  types  of  features: 
bow  shapes,  dark  midlatitude  bands,  and  a 
dark  equatorial  band  (Figure  6-31).  The  dark 
equatorial  band  forms  a  tail  that,  together  with 
a  bow  feature,  produces  the  characteristic 
Y-feature  astronomers  can  see  from  Earth.  This 
feature  appears  clearly  in  the  images  from 
Mariner  10  and  Pioneer  Venus.  At  times,  it  keeps 
its  structure  as  it  moves  around  the  planet, 
showing  a  four-  or  five-day  periodicity.  At 
other  times,  the  Y-feature  is  absent  from  the 
ultraviolet  cloud  patterns.  Even  when  it  is 
present,  many  detailed  features  change  with 
time.  The  variability  of  the  Y-feature  suggests 
that  its  smaller  features  change  independently. 

Cellular  features  with  either  dark  or  bright  sur- 
roundings are  common  at  low  latitudes.  Most 
have  dark  centers.  They  are,  on  the  average, 
about  200  to  300  km  (124  to  186  miles)  in  diam- 
eter and  are  present  in  bright  and  dark  regions. 
They  are  more  numerous  in  the  dark  equatorial 
region  and  during  the  afternoon  on  Venus. 

Figure  6-32.  Pioneer  space- 
craft took  these  eight  consecu- 
tive polar  stereographs  of  Venus' 
northern  hemisphere  at 
11. 5  microns  in  the  infrared 
region  of  the  spectrum.  The 
images  were  taken  one  each  day 
in  orbits  32  through  39 
(January  5  through  12,  1979). 
The  north  pole  is  at  the  center 
of  each  image,  and  the  equator 
is  the  outer  boundary  with  the 
noon  point  at  the  bottom. 
Note  that  the  polar  dipole 
returns  to  about  the  same 
position  in  three  days. 

Ultraviolet  images  from  Orbiter  also  showed 
wave-like  features  about  1000  km  (621  miles) 
long  and  200  km  (124  miles)  apart.  They  made 
large  angles  with  the  equator  and  cut  across 
other  features,  thereby  showing  that  they  were 
at  different  altitudes  from  the  other  features. 

The  Orbiter's  infrared  radiometer  data  showed 
a  dark  polar  band  at  about  65°  to  75°  north 
latitude.  This  broad,  cold  feature  formed  a 
collar  around  the  pole.  It  was  most  likely  an 
unusually  cold  region  near  the  base  of  a 
temperature  inversion.  Its  coldest  part  seemed 
to  follow  the  antisolar  point  around  the  planet. 
Earth-based  observations  indicate  that  polar 
collars  usually  persist  for  weeks  or  months. 
They  are  most  pronounced  near  only  one  pole 
throughout  the  period  when  the  planet  is 
suitably  positioned  for  observation  from  Earth. 

A  localized  polar  brightening  at  very  high 
latitudes  is  generally  associated  with  collars  in 

ground-based  observations.  Pioneer  Venus 
infrared  images  resolved  this  pattern  into  a 
pair  of  "hot  spots"  that  straddle  the  pole. 
These  hot  spots  were  at  about  85°  north  lati- 
tude. They  appeared  as  a  dramatic  "dipole"  in 
images  and  maps. 

Infrared  images  revealed  structure  on  the  night- 
side  as  well  as  polar  regions.  Near  the  pole, 
thin  hazes  and  an  unfavorable  angle  of  solar 
illumination  made  observations  at  other  wave- 
lengths difficult.  The  hot  spots  of  the  polar 
dipole  were  probably  clearings  in  the  polar 
cloud  deck.  This  feature  rotated  about  the  pole 
in  approximately  2.7  days  (Figure  6-32). 
Brightness  temperatures  within  the  hot  spots 
approached  260  K  at  11.5  microns.  But  the 
temperature  could  have  been  as  high  as  280  K 
if  the  spacecraft  had  viewed  the  region  from 
directly  above.  Bright  filamentary  streaks 
emerging  from  one  eye  of  the  dipole  and 
dividing  the  collar  are  visible  in  several  images. 


Figure  6-33.  Infrared 
images  and  plots  of  Venus' 
198       north  polar  regions.  The 
pole  is  at  the  center,  and 
the  outer  boundary  is  50° 
north  latitude.  The  noon 
point  is  at  the  left.  The 
blacked  out  region  indi- 
cates lack  of  data  because 
of  orbit  geometry.  The 
Orbiter  obtained  the  top 
pair  on  orbit  2 1  (Decem- 
ber 26,  1978),  the 
bottom  pair  on  orbit  69 
(February  11,  1979). 

The  dipole  was  about  2000  km  (1243  miles) 
long  and  about  1000  km  (621  miles)  across 
(Figure  6-33).  These  polar  hot  spots  may  be 
evidence  of  atmospheric  subsidence  at  the 
center  of  the  polar  vortex.  Because  descending 
motions  are  not  observed  elsewhere  in  the 
northern  hemisphere  of  Venus,  the  evidence 
points  to  a  single  large  circulation  cell  filling 
the  hemisphere  at  the  level  of  the  cloud  tops. 

Particle  Microphysics 

Size  groupings  distinguish  the  particles  in  the 
main  decks  of  the  upper,  middle,  and  lower 
cloud  regions.  These  groupings  have  more 
than  one  maximum  and  so  are  multimodal.  By 
contrast,  haze  particles  seem  to  group  around 
one  maximum  value  and  are  unimodal. 

Data  from  Pioneer  showed  that,  in  the  lower 
and  middle  cloud  regions,  the  size  distribution 
is  trimodal,  with  modal  diameters  of  0.1  to  0.5, 

Average  upper  cloud  region 
size  distribution 


&^  400 

c  ' 

-S  I  300 

01    I 

1§    200 

3   Z 


0        1234567 
Diameter  (um) 




S'E  20 
3  S-  10 

Average  middle  cloud  region 
size  distribution 

Particle  mass 

5     10  15    20   25    30  35 
Diameter  (um) 

Average  lower  haze  region 
size  distribution 

f.  L 

<u  \ 


0        1234567 
Diameter  (um) 


Average  lower  cloud  region 
size  distribution 

Particle  mass 

5      10  15    20    25    30  35 
Diameter  (um) 

Figure  6-34.  Average  size 
distribution  of  cloud  particles 
from  Large  Probe  data.  A 
multimodal  distribution  is  evi- 
dent, especially  in  the  middle 
and  lower  cloud  regions.  You 
also  can  see  it  in  the  upper  cloud 
layer  and  the  haze  layers.  The 
predoud  region,  part  of  the 
lower  haze,  accounts  for  nearly 
all  the  particles  larger  than 
1.2  microns.  The  mass-relative 
distribution  assumes  that  all 
particles  are  spherical. 

1.8  to  2.8,  and  6  to  9  microns.  The  upper 
cloud  region  is  bimodal,  consisting  of  particles 
from  the  first  two  ranges  of  modal  diameters. 

The  smallest-size  mode  is  a  widespread  aerosol 
population  extending  throughout  the  main 
cloud  deck  and  15  to  20  km  (9.3  to  12.4  miles) 
above  and  below  it.  Its  number  density  varied 
greatly  with  height  but  was  enough  for  the 
particles  to  act  as  centers  for  the  growth  of 
larger  particles.  This  probably  occurs  by  het- 
erogeneous nucleation  from  parent  vapors  in 
the  atmosphere.  The  second-size  mode  consists 
of  droplets  of  sulfuric  acid  with  primary 
growth  taking  place  in  the  upper  cloud  region. 
There  was  a  gradual  increase  in  number 

density  descending  through  the  main  cloud 
deck.  This  mode's  size  distribution  was 
extremely  narrow  at  any  one  altitude,  and  it 
was  the  dominant-size  mode  of  the  planet. 

The  largest-size  mode  for  best  agreement  with 
all  Pioneer  Venus  and  Venera  data  consists  of 
thin  plate-like  crystals.  Their  high  aspect  ratio 
prevented  accurate  determination  of  their 
mass.  Aspect  ratio  is  the  ratio  of  maximum  to 
minimum  projected  area. 

Figure  6-34  shows  the  average  size  distribution 
within  each  cloud  region,  as  measured  by  the 
Large  Probe's  cloud-particle  size  spectrometer. 


The  particle-size  distributions  were  not 
unusual  except  for  the  second-size  mode,  which 
appeared  to  be  monodisperse.  Such  narrow 
distribution  can  be  explained  by  assuming 
competitive  diffusional  growth.  However,  the 
uniformity  of  the  distribution  width  over  the 
planet,  as  hinted  from  the  probe  and  Orbiter 
data,  is  mysterious.  It  is  highly  unlikely  that 
droplets  grow  by  coalescing  because  there  is  low 
probability  of  their  colliding  with  each  other. 

Particle  Composition 

Multimodal  size  distributions  usually  indicate 
several  different  chemical  components  of  a 
population  of  particles.  Scientists  easily  identi- 
fied the  second-size  mode  particles,  mode  2,  as 
sulfuric-acid  droplets.  This  identification  was 
primarily  from  their  optical  properties.  These 
particles  are  traced  throughout  the  main  cloud 
deck.  While  the  concentration  of  sulfuric  acid 
in  the  droplets  may  decrease  from  90%  at  60  km 
(37  miles)  to  80%  at  50  km  (31  miles),  concen- 
tration has  little  effect  on  drop  size. 

The  mode-1  aerosol  is  of  variable  composition 
as  inferred  from  its  optical  properties  and  from 
considerations  of  particle  growth.  The  aerosol  is 
mainly  sulfuric  acid  in  upper  and  lower  cloud 
layers,  precloud  layers,  and  upper  haze  regions. 
Sulfuric  acid  forms  in  the  region  above  the 
boundary  between  the  upper  and  middle 
clouds.  The  mode-1  aerosol  apparently 
contains  other  chemical  species  or  direct 
condensates  as  scavenged  contaminants.  These 
could  account  for  most  of  the  particle  mass 
remaining  in  the  lower  haze  and  perhaps  the 
middle  cloud  regions. 

Composition  of  mode-3  particles  is  uncertain. 
They  may  well  be  chlorides,  but,  if  so,  scien- 
tists have  yet  to  identify  the  cation.  Except 
for  any  particulate  matter  in  the  lower  atmo- 
sphere, essentially  all  particle  mass  is  volatile 
at  temperatures  above  20°C  (68°F).  Venera  11 

instruments  detected  chlorine  in  large  amounts, 
but  its  role  in  cloud  chemistry  is  uncertain. 

Optical  Properties 

The  major  absorption  of  solar  energy  in  Venus' 
atmosphere  takes  place  at  high  altitudes 
corresponding  to  the  locations  of  the  high 
hazes  down  through  the  upper  cloud  regions. 
The  actual  role  of  the  cloud  particles  in  the 
absorption  process  is  not  clear,  but  they 
certainly  play  an  important  role  in  several 
ways.  They  redirect  incident  solar  energy  by 
scattering  processes.  They  increase  the  actual 
absorption  of  incident  photons  in  a  horizontal 
layer  of  the  atmosphere.  They  redirect  most  of 
the  incident  light  into  space. 

Much  of  the  absorption  observed  at  far 
ultraviolet  wavelengths  is  attributable  to 
sulfur-dioxide  vapor.  Measurements  showed 
that  this  gas  wells  upward  in  quantities  that 
match  the  rate  at  which  acid  droplets  of  the 
clouds  settle  downward.  Sulfur  dioxide  is 
oxidized  at  the  cloud  tops  by  ultraviolet 
photochemistry.  It  then  dissolves  in  water 
droplets  to  form  sulfuric  acid.  Infrared  absorp- 
tion is  also  attributed  primarily  to  other 
gaseous  constituents  such  as  carbon  dioxide 
and  sulfuric  acid.  However,  the  absorber  of  an 
important  part  of  the  solar  spectrum  extending 
from  about  3200  angstroms  into  the  visible, 
which  is  also,  in  large  part,  responsible  for  the 
presence  of  the  ultraviolet  markings  observed 
remotely,  has  yet  to  be  identified. 

Since  they  are  transparent  at  the  wavelengths 
involved,  particles  of  pure  sulfuric  acid  do  not 
qualify  as  candidates  for  this  absorption.  There- 
fore, if  the  missing  absorber  is  in  the  particu- 
late matter,  it  must  be  in  the  form  of  a  con- 
taminant or  aerosol  core  to  the  sulfuric-acid 
particles.  Two  factors  point  to  the  location  of 
the  absorber.  First,  the  contrast  of  the  ultravio- 
let features  as  observed  by  Orbiter's  cloud 






5    40 


A   *    A 

Amount  of  Sulfur  dioxide  at  cloudtops 
+     From  spectral  data 
A      From  multicolor  images 




1982  1984 




Figure  6-35.  Investigators 
observed  a  dramatic  decrease  in 
sulfur  dioxide  at  the  cloud  tops 
during  the  Pioneer  Venus 
mission.  In  1 978,  the  ultraviolet 
spectrometer  easily  detected  the 
gas.  Scientists  had  not  detected 
it  before,  even  though  they 
searched  for  decades  from  Earth. 
During  Orbiter's  mission,  the 
amount  of  gas  declined  steadily 
as  the  figure  shows.  Scientists 
suggested  that  a  major  volcanic 
event  had  occurred  on  Venus  just 
before  Orbiter's  discovery  of  the 
gas.  It  would  have  injected  large 
amounts  of  sulfur  dioxide  into 
Venus '  atmosphere. 

photopolarimeter  decreases  as  the  phase  angle 
of  observation  increases.  Second,  the  greatest 
contrasts  appear  when  viewing  normal  to  the 
clouds.  Therefore,  the  ultraviolet  absorber 
must  lie  much  deeper  than  the  overlying  haze. 

However,  data  from  the  Large  Probe's  solar  net 
flux  radiometer  indicated  that  absorption  of 
solar  energy  takes  place  at  altitudes  above 
optical  depths  of  6  or  7.  That  is,  most  absorp- 
tion is  in  or  above  the  upper  cloud  region, 
with  little  absorption  in  the  middle  or  lower 
clouds.  In  addition,  Orbiter  ultraviolet  spec- 
trometer measurements  suggest  that  the 
unknown  absorber's  location  is  connected  with 
the  location  of  the  sulfur-dioxide  absorber. 

Before  Pioneer  Venus,  scientists  had  searched 
in  vain  for  decades  for  the  signature  of  sulfur 
dioxide.  In  1978,  sulfur  dioxide  was  easily 
detected  in  Venus'  atmosphere  (Figure  6-35). 
However,  subsequent  measurements  from 
orbit  showed  a  steady  decline  in  the  amount 
of  sulfur  dioxide.  Scientists  suggested  that  a 
major  volcanic  episode  occurred  early 
in  1978  before  Orbiter's  arrival  at  Venus.  The 

episode  injected  large  amounts  of  sulfur 
dioxide  into  Venus'  atmosphere. 

Several  years  of  observation  by  Orbiter  showed 
that  sulfur  dioxide  in  Venus'  atmosphere 
decreased  to  between  10%  and  29%  of  its 
value  in  late  1978.  One  possible  explanation  is 
that  Venus  is  still  volcanically  active.  Volca- 
noes periodically  inject  massive  amounts  of 
gas  into  the  atmosphere,  and  quantities  of  the 
gas  later  decrease  by  various  processes.  Venus' 
lack  of  water  makes  explosive  volcanic  episodes 
capable  of  pushing  large  amounts  of  sulfur 
dioxide  to  the  cloud  level  unlikely.  There 
might,  nevertheless,  be  episodes  of  low-level 
volcanic  activity  with  the  gases  carried  upward 
by  normal  atmospheric  mixing. 

Fits  of  models  to  data  from  the  Large  Probe's 
solar  flux  radiometer  experiment  suggest  that 
the  imaginary  index  of  refraction  (the  absorp- 
tion portion  of  the  index  of  refraction)  could 
reach  0.05  for  the  mode  1  aerosol.  However, 
correlating  bright  polar  regions  with  large 
amounts  of  cloud  above  the  sulfuric-acid  main 
cloud  at  high  latitudes  argues  for  a  small 


Table  6-2.   Radiative  Properties  of  Cloud  Layers  at  Sounder  Probe  Location 

Cloud  layer  and 
range,  km 

Pressure,  atm 


Fractional  contribution  to  optical  depth 




Mode  2 

Mode  3 

Mode  4 

Upper  haze 
above  75 





Upper  cloud 














































Middle  cloud 




















Lower  cloud 






















Lower  haze 






amount  of  absorption.  Single  scattering 
albedos  (the  ratio  of  the  probability  of  scatter- 
ing to  the  sum  of  the  probabilities  of  scattering 
and  absorption  for  a  single  particle)  range 
from  0.95  (high  absorption)  in  the  upper  cloud 
region  to  0.999  (low  absorption)  in  the  lower 
cloud  region.  The  larger  mode  3  particles  are 
essentially  nonabsorbing.  However,  an 
unknown  mechanism  appears  to  generate 
some  absorption  near  the  boundary  between 
the  upper  and  middle  clouds. 

The  entire  Venus  cloud  system  has  an  optical 
depth  of  25  to  35  at  visible  wavelengths.  That 
is,  the  probability  of  a  single  normally  incident 
photon  passing  through  the  cloud  system 
without  a  single  interaction  with  a  cloud 
particle  is  e~2S  to  e-35.  The  relative  contribu- 
tions of  each  cloud  region  to  the  total  optical 
depth  appear  in  Table  6-2.  Figure  6-36  shows  a 
plot  of  the  clouds'  measured  optical  properties. 

The  radiometric  albedo,  essentially  the  reflec- 
tion coefficient  weighted  over  the  solar  spec- 
trum, is  0.77  to  0.82.  It  increases  from  equator 
to  poles.  The  particle  real  refractive  indices  at 
visible  wavelengths  for  modes  1  and  2  are 
approximately  1.40  to  1.46.  While  consistent 
with  sulfuric  acid,  the  indices  permit  the 
presence  of  many  other  species.  The  real 
refractive  index  of  mode  3  particles  is 
unknown  but  probably  ranges  from  1.5  to  1.7. 
The  imaginary  index  for  these  particles  must 
be  less  than  10'3. 

Dynamical  Processes 

The  cloud  system  is  embedded  in  the  general 
circulation  of  the  atmosphere  at  altitudes  of 
greatest  wind  velocity  and  vertical  wind  shear. 
Atmospheric  motions  consist  mainly  of  a 
zonal  circulation.  The  atmosphere  moves  from 
east  to  west  with  velocities  increasing  from  a 
few  meters  per  second  at  the  surface  to  some- 
times as  high  as  150  m/sec  (490  ft/sec)  at  cloud 













optical  depth 





I        I        I       I        I        I        !        I        I        I        I     •  J        I        I      ll 

0       2 

I I 

6        8     10     12     14     16     18    20     22    24     26     28     30    32     34    36    38     40 
Accumulative  optical  depth 

I        I        I        I        I        I        I        I        I        I        I        I        I        I        I        I 


0       1 


7       8       9     10     11     12     13     14     15    16     17    18 
Downward  solar  flux  (LSFR) 

I I I I I 

19     20 

500  450  400  350  300  250  200  150  100 

Downward  solar  flux  (Wm     ) 

I  I I I I I I I I I I 

35        34  33  32  31  30  29 

Net  solar  flux  (Wm"2) 

I I I I | I 













Nephelometer  backscatter  at  172  (m^Sr"1  X  10"1  ) 

10  20  30  40  50 

Net  long  wave  flux/71  (Wm     ) 




Figure  6-36.  This  figure  sum- 
marizes optical  properties  of 
Venus' cloud  systems.  The  scale 
on  the  right  identifies  the  vari- 
ous regions  of  the  atmosphere. 
Scales  for  the  various  plotted 
curves  appear  below  the  graph. 
When  studied  in  detail,  these 
plots  reveal  a  wealth  of  detail 
about  the  planet's  atmosphere. 


tops.  The  average  cloud  top  velocity  corre- 
sponds roughly  to  the  four-day  circulation. 

Also,  the  data  suggest  a  major,  although  much 
slower,  north-south  circulation  at  several 
meters  per  second.  It  occurs  at  altitudes 

corresponding  to  the  cloud  region.  There 
seems  to  be  atmospheric  movement  from 
equator  to  poles  at  altitudes  corresponding  to 
the  tops  of  the  clouds.  The  movement  subsides 
at  the  poles.  Return  flow  toward  the  equator  is 
at  altitudes  that  match  the  lower  part  of  the 

Figure  6-37.  A  possible 
pattern  for  the  meridional 
circulation  in  the 
atmosphere  of  Venus. 

main  cloud  region.  The  atmosphere  rises  again 
near  the  equatorial  region.  Such  north-south 
cellular  motions  are  called  Hadley  cells.  The 
combination  of  east-west  and  north-south 
motions  produces  vortices  in  the  polar  region. 
These  affect  the  haze  layer  and  produce  an 
apparent  cloud  top  depression  in  the  vortices. 
They  also  might  be  the  reason  for  the  "pileup" 
of  high  latitude  hazes  and  the  even  higher 
latitude  "cold  ring"  observed  by  the  Orbiter's 
instruments.  Figure  6-37  is  a  schematic  drawing 
of  the  suggested  circulation  pattern. 

The  detailed  ultraviolet  and  infrared  features 
observed  from  Earth,  and  from  flyby  and 
orbiting  vehicles,  may  thus  be  in  accord  with 

the  general  behavior  predicted  from  the  in  sitii 
probe  measurements.  Features  involving  the 
four-day  zonal  rotation  are  evident  in  the 
ultraviolet  images.  Most  other  features  result 
from  wave  motions  and  convection  cells 
disturbing  the  level  of  the  upper-altitude 
ultraviolet  absorber.  Thus,  some  features,  such 
as  the  large-scale  Y-shaped  structures,  promi- 
nent at  lower  altitudes,  may  propagate  slowly 
with  respect  to  the  atmosphere.  They  may 
appear  and  disappear  as  the  wave  motion 
dictates.  The  east-west  wind  moves  their  major 
features  around  the  planet.  Smaller 
convection-type  features,  suggesting  rising 
atmosphere  motion,  also  are  evident.  Finally, 
the  suggested  circulation  pattern  may  plausibly 

describe  the  bright  polar  collar,  the  cold  ring, 
polar  hot  spots,  and  infrared  holes. 

Cloud  particle  growth  is  not  strongly  influ- 
enced by  the  large-scale  planetary  circulation. 
Acid  particles  go  along  for  the  ride,  simply 
adjusting  their  acid  concentration  to  each  new 
equilibrium  the  circulation  offers.  Rapid 
circulation  together  with  particle  volatility 
produces  the  planetary  cloud  structures. 

Growth  of  sulfuric-acid  droplets  appears  to  be 
a  very  slow  process  except  in  the  lowest  cloud 
regions.  Recondensation  of  sulfuric  acid  might 
be  quite  rapid  there.  A  large  range  of  particle 
lifetimes  extends  from  years  in  the  upper  hazes 
to  hours  in  the  lower  cloud  region.  Mode-3 
particles  provide  much  of  the  middle  and 
lower  cloud  structure.  Their  growth  starts  near 
the  top  of  the  middle  cloud.  The  particles 
evaporate  at  the  bottom  of  the  lower  cloud. 

As  a  result  of  the  Pioneer  mission,  scientists 
now  have  a  much  better  understanding  of  the 
chemistry  in  Venus'  clouds.  The  clouds  are 
basically  the  product  of  a  cyclical  chemical 
process  involving  elemental  sulfur.  Sulfur 
originates  from  surface  rocks  through  mineral 
buffering  between  the  surface  and  the  atmo- 
spheric gases  of  carbon  dioxide  and  carbon 
monoxide.  Reaction  with  these  gases  produces 
carbonyl  sulfide  and  elemental  sulfur.  The 
carbonyl  sulfide  then  interacts  with  oxygen  in 
a  hot  layer  above  the  surface  to  form  sulfur 
dioxide  and  carbon  monoxide.  High  in  the 
atmosphere  above  the  clouds,  the  sulfur 
dioxide  reacts  with  water  under  the  influence 
of  solar  ultraviolet  radiation  to  produce 
sulfuric-acid  droplets.  After  they  form,  the 
droplets  sink  slowly  toward  the  planet's 
surface.  They  grow  as  they  collide  with  each 
other  and  condense  sulfuric  acid  and  water 
from  the  atmosphere  in  a  condensation  zone. 
Finally,  as  they  fall  toward  the  hot  surface,  the 

lower  atmosphere's  high  temperature  causes 
the  droplets  to  vaporize  and  break  up  into 
sulfur  dioxide  and  water  vapor.  These  chemicals 
then  circulate  in  the  atmosphere  and  continue 
the  process  of  sulfuric-acid  droplet  formation 
above  the  cloud  layers. 

Although  knowledge  about  the  clouds  of 
Venus  has  been  enormously  increased  by  the 
successful  missions  to  the  planet,  there  are  still 
unanswered  questions.  The  identity  of  the 
remaining  ultraviolet  absorber  still  eludes  us. 
Scientists  must  know  what  this  absorber  is  to 
fully  understand  upper  atmosphere  motions 
and  cloud  details.  Also,  this  information  is 
vital  to  understanding  the  planet's  energetics 
and  atmospheric  chemistry.  The  detailed  com- 
position of  mode-3  particles  and  the  nature  of 
contaminants  in  other  cloud  particles  are  still 
in  question.  The  role  of  chlorine  in  cloud 
chemistry  is  unknown.  There  also  are  questions 
about  precipitation  within  the  atmosphere. 
Finally,  we  know  little  about  particles  sus- 
pended in  the  atmosphere  at  low  altitudes,  the 
presence  of  which  is  hinted  at  by  data  from 
several  probe  instruments  and  the  possible 
occurrence  of  lightning  discharges. 


On  Earth,  most  lightning  occurs  in  strongly 
convective  clouds,  but  it  can  also  be  produced 
in  volcanic  clouds,  dust  storms,  and  snow- 
storms. Some  scientists  have  suggested  that  the 
energy  of  lightning  strikes  on  Earth  may  have 
played  an  important  part  in  producing 
complex  molecules  for  the  evolution  of 
terrestrial  life.  Volcanic  eruptions  are  not 
major  producers  of  terrestrial  lightning,  but 
individually  are  very  active  producers.  Over 
the  past  decade,  lightning  has  been  recorded 
optically  on  Jupiter,  Saturn,  Uranus,  and 
Neptune.  These  strikes  have  occurred  mainly 
in  nightside  hemispheres  where  production 
rates  would  be  expected  to  be  lower  than  on 


Figure  6-38.  Lightning  on 
Venus?  Investigators  interpreted 
signals  from  Orbiter's  electric- 
field  detector  as  originating 
from  lightning  in  Venus'  clouds. 
The  conceptual  drawing  shows 
how  the  signals  at  1 00  Hz  and 
higher  frequencies  can  be 


5.4  kHz 


-  730  Hz 



100  Hz 



1936  1937 

Universal  Time 

Low-frequency  whistlers 



Very-high-frequency  whistlers 

Pioneer  Venus 


Veneras  11  and  12 

High-frequency  whistlers 
(fc  <  f  <  f p) 



the  dayside.  Dayside  strikes  can  only  be  "seen" 
as  they  are  recorded  by  electromagnetic 
emissions.  At  Venus,  Pioneer  Orbiter  sought 
evidence  of  lightning  electromagnetically  and 
optically.  The  former  method  was  successful; 
the  latter  was  not. 

Instruments  on  Veneras  11  and  12  observed 
electrical  signals  attributed  to  lightning  on 
Venus  (see  Chapter  7).  The  Pioneer  Venus  orbit- 
ing electric-field  detector  also  observed  signals 
suggestive  of  lightning.  It  recorded  these 
signals  on  its  100  Hz  channel  (Figure  6-38). 
Orbiter  first  recorded  these  whistler-mode  elec- 
tromagnetic noise  bursts  in  December  1978 
when  its  periapsis  moved  from  sunlight  into 
darkness.  It  obtained  more  detailed  observa- 
tions during  Phase  III. 

An  important  observation  was  that  lightning- 
associated  signals  depend  strongly  on  the  local 
time.  As  on  Earth,  high-frequency  signals  do 
not  usually  propagate  far  into  the  ionosphere, 
and  they  clearly  mark  the  surface  region  of 
origin.  The  source  of  the  waves  on  Venus  was 
at  local  times  before  10:30  p.m.  The  decrease 
in  occurrence  as  dusk  approached  was  an 
effect  of  the  increasingly  dense  ionosphere, 
which  altered  propagation  of  the  waves.  Some 
investigators  suggested  that  lightning  probably 
occurs  in  the  afternoon  and  early  evening.  A 
strong  local  time  dependence  suggests  that 
lightning  is  generated  by  weather  conditions 
on  Venus  as  on  Earth.  At  one  time,  however, 
early  in  the  Pioneer  Venus  mission,  scientists 
speculated  that  the  lightning  discharges  were 
related  to  volcanic  activity,  while  other 
scientists  did  not  acknowledge  that  lightning 
was  the  source  of  the  electromagnetic  signals. 

Many  scientists  now  believe  the  signals  do 
originate  from  lightning.  They  cite  four  reasons: 
(1)  the  signals  are  intense  and  highly  impul- 
sive, (2)  they  occur  near  periapsis,  (3)  their 

spectral  characteristics  are  consistent  with 
whistler-mode  propagation,  and  (4)  they  often 
appear  when  low  and  variable  electron 
densities  are  present. 

Known  processes  for  the  formation  of  light- 
ning on  Earth  require  large  particles  and 
strong  updrafts  in  cloud  regions.  Potential 
latent  instability  (the  difference  between  the 
rate  at  which  the  temperature  would  vary  with 
altitude  in  an  idealized  atmosphere  and  the 
actual  lapse  rate)  is  a  measure  of  the  tendency 
of  the  atmosphere  to  overturn  and  undergo 
convective  motion.  Scientists  found  evidence 
of  planet-wide  instability  in  Venus'  middle 
cloud  region.  There,  updrafts  probably  occur 
over  a  limited  altitude  range  from  50  to  56  km 
(30  to  35  miles).  However,  there  is  no  direct 
evidence  for  large  precipitative-type  particles 
similar  to  rain  or  hail.  Thus,  if  cloud  processes 
generate  lightning,  then  large  undetected 
particles  may  exist  in  Venus'  atmosphere. 
Lightning  could  be  the  result  of  local,  large- 
scale  events  such  as  volcanic  eruptions  or 
strong  and  still  undetected  convective  motions 
at  the  subsolar  point.  Also,  because  the  cloud 
base  is  roughly  45  to  50  km  (28  to  31  miles) 
above  the  surface,  lightning  flashes  on  Venus 
would  most  likely  be  from  cloud  to  cloud 
rather  than  from  clouds  to  ground. 

One  goal  during  Orbiter's  entry  phase  was  to 
find  out  if  plasma  wave  signals  could  be 
detected  at  low  altitudes  with  the  spacecraft 
below  the  ionospheric  density  peak.  The  results 
were  positive.  Bursts  of  100  Hz  were  detected 
before  final  entry.  They  were  recorded  at  an 
altitude  of  about  130  km  (81  miles)  around 
4:00  a.m.  local  time.  The  wave  activity  lasted 
for  tens  of  seconds,  and  the  bursts  were  not 
symmetric  about  periapsis.  Their  vertical  atten- 
uation scale  height  of  about  1  km  was  consis- 
tent with  whistler-mode  waves  propagating 
through  the  ionosphere.  Researchers  credited 



the  waves  to  signals  of  electromagnetic 
radiation  entering  the  bottom  of  the  iono- 
sphere from  several  discrete  sources.  This 
would  be  expected  of  lightning  flashes  occur- 
ring frequently  within  Venus'  atmosphere. 

The  data  were  gathered  in  the  predawn  hours, 
but  other  researchers  suggested  that  lightning 
would  be  an  afternoon  or  early  evening  phe- 
nomenon. One  possibility  is  that  entry  phase 
observations  gathered  ambient  wave  noise 
caused  by  lightning  flashes  occurring  at 
locations  remote  from  the  spacecraft.  The 
intense  signals  might  have  propagated  for  con- 
siderable distances  from  their  source  because 
the  planet's  surface  and  ionosphere  acted  as  a 
giant  waveguide.  Investigators  concluded  that 
the  intense  bursts  they  observed  during  the 
final  two  periapsis  passages  were  most  prob- 
ably direct  subionospheric  detection  of  atmo- 
spheric discharges.  In  other  words,  they  were 
lightning.  Some  scientists,  however,  still  linked 
the  wave  bursts  to  local  plasma  instabilities 
and  not  to  lightning  flashes. 

Several  experiments  attempted  to  observe 
lightning  optically  using  the  Pioneer  Venus 
Orbiter's  star  sensor.  They  showed  no  statisti- 
cally significant  difference  in  signals  from  the 
planet's  dark  hemisphere  compared  with  con- 
trol signals  from  pointing  the  star  sensor  into 
deep  space.  These  experiments  thus  implied 
that  most  lightning  occurs  on  Venus'  dayside 
and,  except  for  early  evening,  lightning  would 
be  relatively  rare  on  the  nightside.  The  Venera 
orbiters  did,  however,  obtain  somewhat 
inconclusive  data  that  investigators  attributed 
to  optical  detection  of  lightning  flashes.  (It  is 
important  to  note  that  Pioneer  Orbiter's  star 
sensor  was  not  originally  designed  for  optical 
lightning  detection.) 

It  is  now  generally  accepted  that  nearly 
14  years  of  observations  of  electromagnetic 

signals  from  lightning  at  Venus  indicate  that 
the  flash  rate  is  similar  to  or  greater  than  that 
of  Earth.  More  information,  however,  is 
necessary  before  we  can  speak  with  certainty 
about  the  lightning's  origin  and  any  atmo- 
spheric composition  changes  it  may  cause. 

Atmospheric  Gases — the  Neutral 

An  important  source  of  information  for  the 
way  the  terrestrial  planets — Mercury,  Venus, 
Earth,  and  Mars — formed  is  an  analysis  of  their 
atmospheric  gases.  Scientists  generally  accept 
the  chemical  composition  of  gases  that  formed 
the  primitive  atmospheres  of  these  planets  as 
resembling  that  of  the  Sun  and  the  giant 
planets.  These  gases  were  lost  during  the  early 
stages  of  the  Solar  System's  formation  because 
of  the  high  temperatures  prevailing  at  that 
time.  Scientists  believe  the  present  atmospheres 
consist  of  volatiles  that  were  originally 
incorporated  in  the  solids  that  combined  to 
form  the  planets.  Probably  during  the  first  few 
million  years  in  the  lives  of  these  planets,  high 
internal  temperatures  and  tectonic  activity 
drove  the  volatiles  from  their  crusts  and 
mantles.  Some  of  the  volatiles  make  up  the 
present  atmospheric  gases.  Others,  such 
as  water  vapor,  have  condensed  or  otherwise 
been  transformed.  On  Earth,  water  constitutes 
the  oceans.  On  Mars,  water  is  hidden  below 
the  surface  in  some  form  such  as  permafrost. 
On  Earth,  carbon  dioxide  has  been  converted 
chemically  to  carbonate  rocks  such  as  lime- 
stone. On  Mars,  carbon  dioxide  remains  in 
the  atmosphere  and  in  polar  caps. 

Based  on  this  scenario,  the  amount  of  each 
kind  of  gas  in  the  atmosphere  of  a  terrestrial 
planet  should  depend  mostly  on  the  mass  of 
that  planet.  Studies  of  Mars  by  Mariner  and 
Viking  probes  showed  this  is  not  true.  Even 
allowing  for  its  smaller  size,  Mars  seems  to  be 
deficient  in  volatiles  compared  with  Earth. 

Table  6-3.  Comparison  of  Atmospheres  of  Venus  and  Earth 


Venus  at  surface, 
%  or  ppma 

Earth  at  sea  level, 
%  or  ppma 











Carbon  dioxide 



Carbonyl  sulfide 












Sulfur  dioxidec 



a1  ppm  =  0.0001% 
bDerived  from  36Ar 
c<10  in  clouds;  <300  near  surface 

These  volatiles  include  carbon,  oxygen, 
nitrogen,  and  the  noble  gases  (neon,  krypton, 
and  argon).  Deficiency  factors  are  as  large  as 
100  to  200.  After  the  Viking  mission,  an 
explanation  for  these  results  was  that  the 
material  from  which  Mars  formed  lacked 
volatiles  compared  with  Earth.  Also,  a  smaller 
percentage  of  volatiles  escaped  from  the 
Martian  interior.  The  reason  for  the  deficien- 
cies remains  unknown. 

Because  Earth  and  Venus  are  so  similar  in  size, 
mass,  and  distance  from  the  Sun,  the  volatile 
inventories  of  these  two  planets  were  expected 
to  be  very  similar.  An  exception  was  water 
since  astronomers  knew  Venus  has  no  ocean. 
Hence,  the  stage  was  set  for  the  Pioneer  Venus 
mission  to  conduct  a  crucial  test  of  models  of 
planetary  formation. 

Before  Pioneer  Venus,  scientists  generally 
agreed  that  Venus'  atmosphere  was  mostly 
carbon  dioxide  gas.  Estimates  of  the  fraction 
varied  between  about  95%  and  98%.  Also, 
scientists  believed  that  most  of  the  rest  of  the 

atmosphere  was  nitrogen.  Atmospheric  pres- 
sure on  Earth  is  about  1%  of  that  on  Venus, 
and  carbon  dioxide  makes  up  about  0.03%  of 
Earth's  atmosphere  (Table  6-3).  In  stark  con- 
trast, Venus'  atmosphere  contains  about 
300,000  times  as  much  carbon  dioxide  as 
Earth's.  This  does  not  necessarily  mean  that 
more  carbon  dioxide  has  escaped  into  the  atmo- 
sphere of  Venus  from  its  interior.  The  supply 
of  carbon  in  limestone  rocks  and  elsewhere 
in  the  Earth's  crust  suggests  that  most  of  the 
carbon  dioxide  produced  on  Earth  has  been 
converted  to  carbonates.  A  rough  comparison 
shows  that  Venus  produced  no  more  than 
about  twice  as  much  carbon  dioxide  as  Earth. 
On  Earth,  carbon  dioxide  has  been  incorpo- 
rated in  rocks.  On  Venus,  it  has  remained  in 
the  atmosphere  because  that  planet  lacks  an 
ocean  to  mediate  the  transformation. 

Many  of  the  instruments  carried  by  Orbiter 
were  specially  chosen  to  form  a  synergistic 
package  for  exploring  Venus'  atmosphere  and 
ionosphere.  During  the  years  of  Orbiter's 
mission,  the  spacecraft  gathered  vast  amounts 



of  data  on  the  basis  of  which  scientists  began 
to  understand  the  aeronomy  of  Venus  better 
than  that  of  any  other  planet  except  Earth. 

A  major  problem  faces  scientists  trying  to 
understand  the  divergent  evolutionary  paths 
of  the  two  planets.  How  can  we  account  for 
the  present-day  absence  of  water  on  Venus? 
Was  water  never  present?  Or  had  large 
amounts  of  water  evolved  from  the  interior  at 
an  early  stage  only  to  be  lost  later — hydrogen 
to  space  and  oxygen  to  the  crust  and  interior? 

There  is  another  basic  question  of  importance. 
Can  some  climatic  change  on  Earth,  man- 
made  or  natural,  cause  an  increase  in  carbon 
dioxide  and  water  in  Earth's  atmosphere  that 
results  in  a  runaway  greenhouse?  Because 
carbon  dioxide  and  water  inhibit  the  escape  of 
heat  radiation,  an  increase  in  their  concentra- 
tion would  probably  lead  to  a  rise  in  atmo- 
spheric temperature.  This,  in  turn,  would  lead 
to  the  release  of  more  carbon  dioxide  and 
water  into  the  atmosphere,  and  the  tempera- 
ture would  rise  further,  and  so  on.  The  result 
could  be  an  atmosphere  like  Venus'.  All 
available  carbon  dioxide  might  be  in  the 
atmosphere  and  the  temperature  near  the 
ground  would  approach  700  K  as  on  Venus. 
However,  recent  studies  have  thrown  doubts 
on  the  role  played  by  burning  fossil  fuels  in 
raising  carbon  dioxide  levels  in  our  atmo- 
sphere. Natural  processes  linked  to  solar 
activity  appear  to  play  a  more  dominant  role. 

One  of  the  major  tasks  of  the  instruments  on 
the  Large  Probe,  the  Orbiter,  and  the  Multi- 
probe  Bus  was  to  confirm  that  carbon  dioxide 
and  nitrogen  are,  indeed,  the  main  atmo- 
spheric constituents  on  Venus,  and  to  deter- 
mine their  precise  concentrations.  The  instru- 
ments also  had  to  identify  other  atmospheric 
components,  even  if  these  were  only  one  part 
per  billion  (1  ppb).  Instruments  on  the  Large 

Probe  that  were  assigned  to  these  tasks  were 
the  neutral  mass  spectrometer  and  the  gas 
chromatograph.  The  mass  spectrometer 
covered  altitudes  from  62  km  (39  miles)  to  the 
surface.  The  gas  chromatograph  sampled  the 
atmosphere  at  52,  42,  and  22  km  (32,  26,  and 
13.7  miles).  On  the  Bus,  a  mass  spectrometer 
obtained  data  above  130  km  (81  miles).  On  the 
Orbiter,  another  mass  spectrometer  sampled 
the  atmosphere  above  145  km  (90  miles). 
Important  data  about  atmospheric  composi- 
tion above  the  clouds  also  came  from  the 
Orbiter's  ultraviolet  spectrometer. 

Scientists  have  reached  a  consensus  over 
measurements  from  the  Pioneer  instruments 
and  from  those  on  the  Veneras  11  and  12 
landers.  They  now  agree  Venus'  atmosphere  is 
96%  carbon  dioxide  and  4%  nitrogen.  Its  sur- 
face pressure  is  94.5  times  that  of  Earth  and  its 
temperature  is  732  K.  These  figures  mean  that 
Venus  has  outgassed  1.8  times  as  much  carbon 
dioxide  as  Earth  and  2.3  to  4  times  as  much 
nitrogen.  The  nitrogen,  however,  depends  on 
how  much  is  still  in  the  Earth's  crust.  Thus, 
the  expectation  was  confirmed  of  a  rough 
equality  in  the  volatiles  of  Earth  and  Venus  for 
carbon  dioxide  and  nitrogen. 

However,  an  assay  of  the  remaining  volatiles 
in  Venus'  atmosphere  delivered  a  rude  shock 
to  the  planetary  science  community.  The  case 
of  argon  is  an  example.  Two  isotopes  of  argon 
are  of  interest  to  scientists  studying  planetary 
atmospheres.  Radiogenic  argon-40,  the  most 
abundant  kind  of  argon  in  Earth's  atmosphere, 
is  produced  by  the  radioactive  decay  of 
potassium.  Its  abundance  tells  us  about  the  prim- 
itive concentration  of  potassium  and  about 
outgassing  conditions  throughout  the  planet's 
4.5-billion-year  history.  On  the  other  hand, 
argon-36  and  argon-38  are  primordial  gases 
which  tell  us  about  the  early  volatile  content 
of  planetary  interiors  and  how  they  outgassed. 

Table  6-4.   Mixing  Ratios  in  the  Lower  Atmosphere 


Amount,  ppm 




Carbon  dioxide 
Carbon  monoxide 
Nitrogen  (percentages) 

Sulfur  dioxide 

40-1 20 







3.41  %  (at  24  km)a;  4%b 
3.54%  (at  44  km)a 
4.60%  (at  54  km)a 

1 6  (at  44  km)a;  <30b 
43  (at  55  km)a 

1 85  (at  24  km) 
<1 0  (at  55  km) 

20  (at  surface) 
60-1 350  (at  24  km) 
1 50-5200  (at  44  km) 
200-<600  (at  54  km) 

aLarge  Probe  Gas  Chromatograph 
bl_arge  Probe  Mass  Spectrometer 

Based  on  six  different  instruments 

-four  mass  spectrometers  and  two  gas  chromatographs. 

On  the  basis  of  carbon  and  nitrogen  results, 
scientists  expected  that  there  would  be  about 
as  much  argon-36  and  argon-38  in  Venus' 
atmosphere  as  in  Earth's  atmosphere.  Instead, 
the  mass  spectrometers  on  the  Pioneer  and 
Venera  landers  found  about  equal  concentra- 
tions of  radiogenic  argon-40  and  nonradiogenic 
argon.  About  30  atoms  in  every  million  atmo- 
spheric molecules  (30  ppm)  were  argon-36.  The 
gas  chromatograph,  which  could  not  distinguish 
among  argon's  various  isotopes,  supported  the 
mass  spectrometer  results  (Table  6-4).  Their  data 
showed  a  total  concentration  between  50  and 
70  ppm.  Since  Venus'  atmosphere  contains  about 
75  times  as  many  molecules  as  Earth's,  it  con- 
tains 75  times  as  much  argon-36  as  Earth's  atmo- 
sphere. Yet,  the  ratio  of  argon-38  to  argon-36 
is  almost  identical  to  the  terrestrial  ratio. 

One  discordant  note  came  from  the  Bus'  neu- 
tral mass  spectrometer.  It  could  not  detect  argon 
at  130  km  (81  miles).  By  extrapolation  to  the 
lower  atmosphere,  this  result  would  imply  that 
there  is  less  than  10  ppm  of  argon-36  in  Venus' 

atmosphere.  Even  this  upper  limit,  however, 
does  not  exclude  the  possibility  of  25  times  as 
much  argon-36  as  in  Earth's  atmosphere. 

Examination  for  neon,  another  primordial 
rare  gas,  confirmed  the  argon  story.  Pioneer 
instruments  and  Venera's  neutral  mass 
spectrometers  placed  the  abundance  of  neon 
between  about  4  and  1 3  ppm  compared  with 
18.2  ppm  for  Earth.  The  ratio  of  neon-22  to 
neon-20  was  0.07.  Compared  with  the  argon 
isotopes,  this  ratio  is  lower  than  the  value  on 
Earth  (about  0.1),  but  is  close  to  the  solar  ratio. 

The  notion  that  Venus,  Earth,  and  Mars 
formed  from  materials  containing  the  same 
endowment  of  volatiles,  already  shaken  by  the 
Viking  results,  was  completely  refuted  by  the 
data  from  Pioneer  Venus.  Why  should  Venus 
have  received  about  twice  as  much  carbon 
dioxide  and  nitrogen  as  Earth?  And  why  does 
it  have  about  50  to  100  times  as  much  neon 
and  nonradiogenic  argon? 



After  a  review  of  early  data  from  Pioneer  and 
Venera  missions,  researchers  suggested  a  possi- 
ble reason.  The  planets,  they  hypothesized, 
formed  from  dust  grains  in  the  solar  nebula. 
These  grains  were  surrounded  by  gas  at  a  pres- 
sure that  diminished  rapidly  with  increasing 
distance  from  the  center  of  the  nebula.  Reactive 
volatiles  such  as  carbon,  nitrogen,  and  oxygen 
would  be  chemically  combined  within  the 
grains.  Rare  gases  would  be  adsorbed  from  the 
surrounding  gas  in  amounts  depending  on  the 
pressure.  As  a  result,  grains  forming  the  three 
planets  would  possess  about  the  same  reactive 
volatile  content,  while  the  rare  gas  concentra- 
tion would  decrease  rapidly  with  increasing 
distance  from  the  Sun.  This  model  required 
that  the  nebula's  gas  temperature  should  be 
fairly  constant.  Also,  early  outgassing  from 
Mars  should  be  less  efficient  by  a  factor  of  20 
than  from  the  other  two  planets. 

Analysis  of  the  Large  Probe's  neutral  mass 
spectrometer  data  produced  another  surprise. 
Although  Venus'  atmosphere  contains  a  large 
excess  of  neon  and  primordial  argon,  this  is 
not  so  with  two  other  rare  gases.  The  absolute 
abundance  of  krypton  is  only  about  three 
times  larger  in  Venus'  atmosphere  than  in 
Earth's.  There  is  much  less  than  30  times  more 
xenon.  In  the  grain  accretion  model,  there  is 
no  reason  to  expect  enrichment  of  one  rare  gas 
to  be  greater  than  another.  In  fact,  a  close  look 
at  Mars  data  shows  that,  from  Mars  to  Earth, 
the  enrichment  decreases  from  a  factor  of 
about  220  for  neon,  through  165  for  argon 
and  110  for  krypton,  to  30  for  xenon. 

Another  way  to  look  at  these  results  is  to 
compare  the  ratio  of  primordial  argon  to 
krypton  on  the  terrestrial  planets  with  the 
ratio  on  the  Sun.  The  ratio  is  4000  in  the  Sun's 
atmosphere,  1000  on  Venus,  50  on  Earth,  and 
40  on  Mars.  So,  the  ratio  gets  more  solar-like 
the  closer  the  planet  is  to  the  Sun.  This 

suggests  that  perhaps  the  material  that 
accreted  to  form  the  planets  was  exposed  to  a 
strong  irradiation  by  gas  of  solar  composition 
flowing  away  from  the  Sun  as  the  Solar  System 
formed.  If  so,  the  grains  and  small  bodies  that 
formed  the  planets  would  have  volatiles  from 
the  Sun  in  addition  to  those  from  the  nebular 
gas  in  their  neighborhood.  The  material 
forming  Venus  may  have  received  a  larger 
share  of  solar  gases  than  the  other  planets.  In 
intercepting  much  solar  gas,  the  material 
forming  Venus  would  have  shielded  the  outer 
regions  of  the  Solar  System  from  this  gas. 

Another  possibility  is  that  Mars  formed  earlier 
than  Earth,  and  Earth  much  earlier  than 
Venus.  This  would  explain  why  Mars  lost  most 
of  its  volatiles.  The  planet  may  have  originated 
early  enough  to  have  retained  such  highly 
radioactive  substances  as  aluminum-26  left 
over  from  a  nearby  supernova  explosion 
believed  to  have  triggered  the  formation  of  the 
solar  nebula.  The  heat  produced  by  the  decay 
of  this  radioactive  aluminum  might  have 
driven  off  many  of  the  Martian  volatiles 
very  early. 

Two  important  noble  gases  are  produced  by 
radioactive  decay  of  heavy  elements  such  as 
uranium.  One  is  argon-40,  the  other  is  helium-4. 
The  consensus  regarding  Pioneer  Venus  and 
Venera  measurements  is  that  argon-40  and 
argon-36  are  about  equal  in  abundance  on  Venus. 
On  Earth,  argon-40  is  about  400  times  as 
abundant  as  argon-36.  Since  there  is  75  times 
as  much  argon-36  on  Venus  as  on  Earth,  this 
means  there  is  only  about  one-fourth  as  much 
argon-40  on  Venus  as  on  Earth.  Venus  either 
started  with  much  less  potassium  than  Earth 
or  is  yielding  up  its  argon  from  the  interior 
more  slowly  than  is  Earth.  Several  factors  may 
account  for  a  slow  escape  of  gases  during 
Venus'  4.5  billion  year  lifetime.  These  factors 
include  lack  of  widespread  tectonics,  a  thicker 

and  relatively  plastic  unfractured  lithosphere, 
and  absence  of  surface  erosion  by  water. 

Measurement  of  helium  in  the  upper  atmo- 
sphere by  the  Bus'  neutral  mass  spectrometer 
agrees  with  this  picture.  Extrapolation  to  the 
lower  atmosphere  suggests  that  there  are 
about  12  helium  atoms  per  million  molecules 
in  the  planet's  atmosphere.  This  works  out  to 
an  absolute  abundance  of  helium  on  Venus 
250  times  greater  than  on  Earth.  Yet,  we  can- 
not conclude  that  Venus  has  vented  that  much 
more  helium-4.  Scientists  know  that  the 
present  atmospheric  amount  of  helium  would 
be  produced  by  radioactivity  in  Earth's  interior 
in  about  one  million  years.  Earth's  atmosphere 
is  losing  helium  at  a  great  rate.  The  amount 
actually  produced,  vented,  and  lost  is  at  least 
10,000  times  what  now  remains.  The  best  esti- 
mate is  that  5  to  10  times  as  much  helium  has 
been  produced  and  escaped  from  Earth's  atmo- 
sphere compared  with  Venus.  Hence,  ineffi- 
cient present  release  of  gas  from  Venus'  inte- 
rior may  account  for  the  difference  between 
the  radiogenic  gas  inventories  of  the  two 
planets,  if  they  contain  equivalent  amounts  of 
potassium  and  uranium. 

The  amount  of  water  vapor  present  in  an  atmo- 
sphere has  important  implications  for  the 
atmosphere's  temperature  structure.  Water 
vapor  plays  a  major  role  in  the  greenhouse 
mechanism  invoked  to  account  for  the  very  high 
temperature  of  the  atmosphere  near  Venus' 
surface.  It  also  has  an  important  bearing  on 
the  chemical  composition  of  the  atmosphere. 

Unfortunately,  accurately  measuring  the 
amount  of  water  vapor  in  an  atmosphere  is 
very  difficult.  Scientists  are  even  uncertain 
about  the  exact  amount  of  water  in  Earth's 
stratosphere.  After  the  Venus  Multiprobe 
mission  of  1979,  a  similar  state  of  confusion 
developed  about  the  amount  of  water  vapor  in 

Venus'  atmosphere.  Data  from  the  Large 
Probe's  neutral  mass  spectrometer  showed  less 
than  0.1%  water  in  the  atmosphere.  A  special 
optical  device  on  the  Venera  probes  found  a 
small  amount,  too.  Its  measurements  suggested 
that  water  decreases  from  200  ppm  at  50  km 
(31  miles)  to  20  ppm  at  the  surface.  On  the 
other  hand,  the  probe's  gas  chromatograph 
data  showed  0.52%  of  water  at  42  km  (26  miles) 
and  0.13%  at  22  km  (13.7  miles).  These  were 
much  greater  amounts. 

The  amount  of  carbon  monoxide  gas  in  Venus' 
atmosphere  is  minute.  According  to  data  from 
the  gas  chromatograph,  its  concentration  is 
about  20  ppm  at  22  km  (13.7  miles).  At  the 
cloud  tops,  it  is  about  50  ppm  as  deduced  from 
Earth-based  observations.  If  carbon  monoxide 
is  produced  by  photodissociation  of  carbon 
dioxide  above  the  clouds  and  subsequently 
diffuses  downward,  this  kind  of  distribution 
would  result.  However,  the  amount  of  carbon 
monoxide  expected  to  accompany  carbon 
dioxide  as  it  vents  from  a  planet's  interior  is 
far  greater  than  the  amount  observed  on 
Venus.  At  least  a  thousand  times  as  much 
carbon  monoxide  should  have  been  produced. 
It  is  conceivable  that  carbon  monoxide  may 
have  reacted  with  water  to  form  hydrogen  and 
carbon  dioxide  early  in  the  planet's  history. 
This  explanation  could  account  for  the  lack  of 
water  on  Venus.  Hydrogen  might  have  escaped 
into  space.  However,  it  is  most  unlikely  that 
the  initial  amounts  of  water  and  carbon  mon- 
oxide were  so  nearly  equal  that  they  would 
have  mutually  reduced  each  other  to  such 
minor  quantities  as  are  now  on  the  planet. 

Oxygen  is  one  of  the  other  constituents  found 
by  various  instruments.  This  gas  increases  from 
16  ppm  to  43  ppm  between  42  and  52  km 
(26  and  32  miles),  according  to  data  from  the 
gas  chromatograph.  The  Large  Probe's  neutral 
mass  spectrometer  produced  data  that  show 


the  amount  of  oxygen  as  less  than  30  ppm. 
Earth-based  measurements  find  less  than  1  ppm 
at  the  cloud  tops.  The  coexistence  of  carbon 
monoxide  and  molecular  oxygen  in  the  atmo- 
sphere is  difficult  to  understand  thermody- 
namically.  Photolysis  of  carbon  dioxide  above 
the  clouds  would  form  oxygen  along  with 
carbon  monoxide.  These  should  decrease  in 
abundance  downward.  However,  the  amounts 
researchers  found  below  52  km  (32  miles)  were 
quite  inconsistent  with  the  small  amount  they 
observed  above  the  clouds.  Thus,  the  oxygen 
measurements  presented  an  enigma. 

Carbon  dioxide,  which  makes  up  the  bulk  of 
Venus'  atmosphere,  is  mysteriously  stable. 
Orbiter  found  the  reason.  In  the  highest  part 
of  the  atmosphere,  rapid  decomposition  of 
carbon  dioxide  by  sunlight  is  an  ongoing  pro- 
cess, releasing  heat  that  drives  the  planetwide 
system  of  winds.  These  winds  blow  from 
dayside  to  nightside.  On  the  nightside,  winds 
descend  to  lower  altitudes  carrying  the  carbon 
monoxide  and  oxygen  down  with  them.  A 
striking  confirmation  of  the  process  came  from 
several  ultraviolet  spectrometer  images  that 
showed  atoms  of  nitrogen  from  the  dayside 
"burning"  on  the  nightside  to  produce  an 
ultraviolet  "flame."  This  occurred  in  regions 
low  enough  for  sufficient  oxygen  pressure  to 
support  the  reaction.  Calculations  show  that 
once  the  dissociated  gases  reach  lower  alti- 
tudes, they  reform  carbon  dioxide  under  the 
influence  of  chlorine-catalyzed  photochem- 
istry above  the  cloud  tops.  The  carbon  dioxide 
is  recycled  with  the  result  that  the  bulk  of  the 
atmosphere  remains  stable. 

Among  sulfur  compounds,  the  measurements 
would  allow  no  more  than  3  ppm  of  the  inter- 
esting molecule  carbonyl  sulfide.  Yet,  sulfur 
dioxide  appears  to  be  present  near  22  km 
(13.7  miles)  in  the  fairly  large  amounts  of  130  to 

185  ppm.  Above  the  clouds,  the  amount  is 
only  0.1  ppm.  Finally,  the  neutral  mass  spec- 
trometer detected  hydrogen  sulfide  gas  with  a 
mixing  ratio  decreasing  from  about  3  ppm  at 
the  surface  to  1  ppm  in  the  clouds.  These 
results  have  an  important  bearing  on  the  ques- 
tion of  how  Venus'  clouds  form.  We  know  the 
clouds  contain  large  amounts  of  sulfuric  acid. 
Before  Pioneer  Venus,  scientists  suggested  a 
cycle  of  chemical  reactions  similar  to  one 
responsible  for  formation  of  sulfate  aerosol 
layers  on  Earth.  In  this  cycle,  carbonyl  sulfide 
plays  a  key  role.  Failure  to  find  carbonyl 
sulfide  in  Venus'  atmosphere  was  a  major 
surprise.  Now  scientists  are  considering 
mechanisms  that  use  a  sulfur  dioxide  and 
water  source  to  produce  the  sulfuric  acid. 

Upper  limits  for  other  important  species  have 
been  set  by  data  from  the  gas  chromatograph. 
These  are  10  ppm  for  hydrogen,  1  ppm  for 
methane,  and  1  ppm  for  ethylene. 

The  neutral  mass  spectrometer  made  atmo- 
spheric composition  measurements  during  the 
final  entry  phase  of  the  Orbiter's  mission.  The 
entry  data  at  lower  solar  activity  filled  a  gap  in 
the  midnight  to  5:00  a.m.  local  solar  time 
range  of  the  earlier  data  gathering  period  of 
1978-1980.  The  earlier  data  extended  to  140  km 
(87  miles)  from  midnight  to  1:00  a.m.  and 
were  above  155  km  (96  miles)  from  2:00  to 
4:30  a.m.  The  entry  data  extended  down  to 
130  km  (81  miles)  during  the  same  local  times. 
On  the  last  orbit,  the  spacecraft  obtained  data 
down  to  128.8  km  (80  miles).  In  Phase  III,  data 
were  gathered  about  helium  above  1 70  km 
(106  miles)  from  6:00  p.m.  to  midnight  local 
solar  time.  Also,  from  midnight  to  4:30  a.m. 
below  200  km  (124  miles),  Orbiter  gathered 
data  on  helium,  atomic  nitrogen,  atomic  oxy- 
gen, carbon  monoxide,  molecular  nitrogen, 
and  carbon  dioxide. 



108  F 





i      i      i 

i      l      i      i      i 

i      l      i 

Local  solar  time  (hr) 


i      l 


Figure  6-39.  Measurements  of 
helium  number  densities  from 
1 70  km  (1 05  miles)  altitude  over 
three  of  Venus'  diurnal  cycles 
(each  of  225  Earth  days).  The 
letters  a,  b,  and  c  identify  these 
cycles.  Note  how  they  are  very 
much  the  same  over  the  cycle  of 
solar  activity. 

During  Phase  III  of  the  mission,  helium  was  the 
dominant  species  in  the  postmidnight  sector 
above  170  km  (106  miles).  The  number  den- 
sities of  helium  at  an  altitude  of  170  km 
(106  miles)  over  three  diurnal  cycles  of  the 
Pioneer  mission  appear  in  Figure  6-39.  The  fig- 
ure shows  the  three  cycles  separately  and  iden- 
tifies them  as  a,  b,  and  c.  Very  little  change  is 
apparent  over  the  solar  activity  cycle. 

Also  in  Phase  III,  oxygen  was  the  dominant 
species  from  140  to  170  km  (87  to  106  miles). 

Carbon  dioxide  was  dominant  below  140  km 
(87  miles).  Estimated  scale  height  temperatures 
for  helium,  oxygen,  and  carbon  dioxide  were 
about  105  to  120  K.  This  was  similar  to  tem- 
peratures researchers  observed  in  1978-1980  at 
a  period  of  higher  solar  activity.  The  diurnal 
variation  of  exospheric  temperature,  based  on 
number  densities  and  scale  heights  over  one 
sidereal  period  of  225  Earth  days  early  in  the 
mission,  appears  in  Figure  6-40. 



2-  ^o 

=    300 


|-  200 



•      4 
•  If 

> . 


i   i   i   i   I   i   i   i   i   i   I 



12  18  24  6 

Local  solar  time  (hr) 





Figure  6-40.  Diurnal  variation 
of  exospheric  temperature. 
Researchers  derived  these  data 
from  number  densities  and  scale 
heights  that  Orbiter's  neutral 
mass  spectrometer  measured. 
The  data  cover  one  diurnal 
cycle  of  Venus  that  lasted 
225  Earth  days. 


The  densities  at  1:00  a.m.  local  solar  time  and 
at  150  km  (93  miles)  altitude  were  within  35% 
of  earlier  measurements.  These  measurements 
occurred  during  the  concluding  phase  of  the 
mission.  Also,  the  helium  bulge  was  similar  to 
that  in  1978-1980.  This  confirmed  that  super- 
rotation  of  the  thermosphere  was  still  occur- 
ring. It  appeared  that  small  changes  in  the 
dayside  thermosphere  arising  from  changes  in 
solar  activity  have  little  impact  on  the  night- 
side  thermosphere.  The  densities  at  an  altitude 
of  170  km  (106  miles)  for  carbon  dioxide  and 
atomic  oxygen  plotted  against  local  solar  time 
appear  in  Figure  6-41.  Orbiter  obtained  these 
over  almost  three  sidereal  days,  and  they 
showed  little  change  over  the  solar  cycle. 

Water-vapor  measurements  presented  major 
theoretical  problems.  Use  of  the  high  value 
obtained  from  the  Pioneer  Venus  gas  chromato- 
graph  in  a  thermodynamic  calculation  created 
an  anomaly.  It  predicted  amounts  of  hydrogen 
sulfide  and  carbonyl  sulfide  somewhat  larger 
than  the  gas  chromatograph  itself  would 
allow,  but  consistent  with  the  mass  spectrom- 
eter measurements.  The  smaller  amount  that 
the  Venera  photometer  found  would  not  allow 
nearly  so  much  hydrogen  sulfide  as  the  mass 
spectrometer  found.  On  the  other  hand,  an 
elementary  conservation  law  states  that  the 
ratio  of  hydrogen  atoms  to  the  total  number 
of  gas  molecules  of  all  kinds  must  remain 
constant  in  the  atmosphere  below  the  clouds. 

Whether  the  gas  chromatograph  measurement 
of  0.52%  water  at  52  km  (32  miles)  or  the 
photometer  value  of  200  ppm  is  correct,  com- 
pounds with  equivalent  amounts  of  hydrogen 
atoms  must  exist  at  the  surface.  Their  concen- 
trations must  vary  to  keep  the  hydrogen 
mixing  ratio  constant.  Scientists  could  not 
find  these  hydrogen  compounds.  Thus,  hydro- 
gen presents  a  continuing  dilemma  as  it  gen- 
erally does  in  studies  of  planetary  atmospheres. 

An  important  question  for  many  reasons  is 
whether  the  atmosphere  is  reducing  or 
oxidizing.  Scientists  are  sure  that  it  is  very 
close  to  the  dividing  line  between  these  two 
states  but  are  still  unsure  as  to  which  side  it  is 
on.  The  amount  of  carbon  monoxide  detected 
seems  to  be  slightly  greater  than  the  amount  of 
molecular  oxygen.  Some  scientists  doubt  the 
presence  of  the  latter.  Thus,  a  case  can  be  made 
that  Venus'  atmosphere  is  in  a  reducing  state. 

Orbiter's  instruments  recorded  wave-like 
perturbations  in  the  nightside  neutral  atmo- 
sphere. These  were  interpreted  as  being  due  to 
gravity  waves  penetrating  upward  from  the 
lower  thermosphere.  Gravity  waves  couple  the 
upper  atmosphere  to  the  lower  thermosphere 
and  modify  the  circulation  of  the  lower  thermo- 
sphere. Researchers  suggest  that  these  gravity 
waves  couple  the  lower  atmosphere  super- 
rotation  at  the  cloud  tops  to  the  superrotation 
in  the  thermosphere.  The  latter  was  inferred 
from  measurements  of  the  neutral  composition. 

The  neutral  atmosphere,  unlike  the  iono- 
sphere, showed  very  little  variability  from  solar 
maximum  to  solar  intermediate  conditions. 
This  was  especially  true  on  the  nightside, 
probably  because  the  nightside  neutral  atmo- 
sphere is  insulated  from  solar  cycle  dependent 
changes  in  the  dayside  thermosphere. 

Aurora  andAirglow 

An  unexpected  discovery  by  Orbiter  was  the 
presence  of  ultraviolet  emissions  from  oxygen 
in  the  high  nightside  atmosphere.  Researchers 
explained  these  emissions  as  being  due  to 
energetic  particles,  either  electrons  or  ions, 
entering  the  atmosphere  from  space.  Such 
emissions  are  common  on  planets  that  have 
magnetic  fields.  The  particles  originate  in  the 
solar  wind  and  travel  along  magnetic  field 
lines  toward  the  planet's  magnetic  polar 
regions.  On  Earth,  we  call  the  emissions  aurora 
borealis  and  aurora  australis,  or  northern  and 
southern  lights. 

However,  Venus  has  no  intrinsic  magnetic 
field  to  trap  and  direct  the  solar  wind's  charged 
particles.  The  origin  of  the  particles  responsible 
for  Venus'  aurora  is  uncertain.  Because  the 
brightness  of  emissions  from  Venus'  atmo- 
sphere is  related  to  solar  activity,  scientists  sug- 
gested one  possible  reason.  The  particles  may 
be  photoelectrons  that  extreme  ultraviolet 
solar  radiation  produces  on  the  dayside.  Weak 
but  turbulent  magnetic  fields  produced  by  the 
action  of  solar  wind  on  Venus'  ionosphere 
could  carry  them  into  the  night  hemisphere. 

Observation  of  high-altitude  airglow  at  the 
limb  was  an  important  discovery  by  Mariners  5 
and  10.  Pioneer  Venus  Orbiter  added  informa- 
tion about  this  phenomenon.  All  three 
spacecraft  observed  Lyman  alpha  radiation, 
which  is  a  tracer  for  hydrogen  atoms.  Also, 
Mariner  10  provided  data  for  helium  that 
allowed  the  first  unambiguous  determination  of 

the  temperature  of  Venus'  exosphere.  The 
exosphere  was  found  to  have  a  probable 
temperature  not  of  700  K  as  previously 
supposed,  but  one  of  only  350  K. 

Among  the  light  atmospheric  elements  that 
travel  to  the  nightside  are  oxygen  and  nitro- 
gen atoms,  produced  on  the  dayside  by  solar 
ultraviolet  radiation.  There  is  a  nightside  bulge 
of  atomic  oxygen  (Figure  6-41).  When  the 
atoms  are  carried  down  again  into  lower  atmo- 
spheric regions,  they  recombine  into  oxygen 
molecules  and  nitric  oxide.  In  so  doing  they 
emit  airglows.  Mapping  of  the  nitric  oxide 
airglow  as  observed  by  the  Orbiter  reveals  a 
concentration  near  2:00  a.m.  local  time. 

Oxygen  glows  in  the  infrared  and  visual 
regions  of  the  spectrum.  The  infrared  glow  had 
been  observed  but  not  mapped  from  Earth. 
Venera  15  mapped  the  glow  in  the  visual 
region,  but  its  variations  are  more  subdued 
than  the  nitric  oxide  glow. 

The  Ionosphere 

Data  from  Pioneer  Venus  greatly  increased  our 
knowledge  of  Venus'  ionosphere.  The  iono- 
sphere of  a  planet  is  a  region  of  the  upper 
atmosphere  with  a  high  density  of  electrically 
charged  particles — electrons  and  ions.  These 
charged  particles  are  usually  a  product  of 
extreme  ultraviolet  solar  radiation  interacting 
with  neutral  molecules  and  atoms  of  the  upper 
atmosphere.  The  types  and  densities  of  ions  in 
an  ionosphere  depend  on  the  neutral  composi- 
tion, the  chemical  reactions  that  occur,  and 
how  the  ions  move  from  place  to  place  within 
the  ionosphere.  Magnetic  fields  affect  the 
behavior  of  a  gas  consisting  of  charged 
particles  (known  as  a  plasma). 

Measurements  of  the  delay  time  in  the  arrival 
of  a  radio  wave  passing  from  a  spacecraft  to 
receiving  stations  on  Earth  provides 


Figure  6-4 1 .  Measurements  of 
carbon  dioxide  and  atomic 
oxygen  number  densities  at 
170  km  (105  miles)  altitude. 
Orbiter's  mass  spectrometer 
recorded  them  over  nearly  three 
diurnal  cycles  (about  675  Earth 
days).  Note  the  smaller  peak  of 
oxygen  to  the  right  of  the 
dayside  curve. 







10  ,- 









I          I          I          I  i_ 


«•  ''  ^ 





•    • 


Local  solar  time  (hr) 




information  on  the  electron  densities  encoun- 
tered along  the  way.  Experimenters  used  this 
technique  to  obtain  information  concerning 
the  ionosphere.  They  arranged  for  the  radio 
waves  to  pass  through  Venus'  atmosphere  on 
their  way  to  Earth  as  the  spacecraft  went  into 
and  emerged  from  occultation  by  Venus.  On 
earlier  flyby  and  orbital  missions  before 
Pioneer,  this  technique  obtained  some  limited 
data  on  the  total  electron  densities.  Pioneer 

Venus  Orbiter  not  only  employed  this  tech- 
nique but  also  made  the  first  in  sitii  measure- 
ments of  Venus'  ionosphere.  The  spacecraft 
used  the  following  instruments:  an  ion  mass 
spectrometer,  a  Langmuir  probe,  a  retarding 
potential  analyzer,  and  a  fluxgate  magnetom- 
eter. Information  from  these  instruments 
helped  develop  a  picture  of  global  composition 
and  dynamics. 

On  Venus,  the  ionospheric  electron  density 
reaches  a  maximum  at  altitudes  near  140  km 
(87  miles).  This  occurs  on  both  the  dayside 
and  the  nightside.  Generally,  Orbiter  could 
not  directly  access  this  level  because  it  is 
slightly  below  the  lowest  periapsis  altitude  the 
spacecraft  reached.  Scientists,  however,  were 
able  to  examine  this  density  maximum  with 
the  radio  occultation  technique.  Above  this 
density  peak,  the  electron  density  decreases 
gradually  with  increasing  height.  In  regions 
directly  accessible  to  Orbiter's  instruments,  the 
Langmuir  probe  made  high  time-resolution 
measurements  of  both  the  electron  density  and 
temperature  that  revealed  many  unusual  iono- 
spheric events  (Figure  6-42).  These  included 
ionospheric  density  depletions  ("holes")  and 
detached  plasma  clouds.  Also,  Orbiter's  ion 
mass  spectrometer  measured  plasma  composi- 
tion and  its  total  density  for  the  first  time. 
More  data  on  plasma  composition  came  from 
the  retarding  potential  analyzer  and  from 
measurements  of  ion  temperature,  photoelec- 
tron  fluxes,  and  plasma  drifts. 

Earth's  ionosphere  reaches  heights  of  many 
thousand  kilometers,  gradually  tapering  off 
with  increasing  altitude.  This  high  altitude 
extension  is  possible  because  a  strong  intrinsic 
dipole  magnetic  field  shields  Earth's  iono- 
sphere from  the  solar  wind.  By  contrast, 
Venus'  intrinsic  magnetic  field  is  negligible 
and  the  solar  wind  interacts  directly  with  the 
ionosphere.  Venus'  ionosphere  is  an  obstacle 
to  the  solar  wind  and  deflects  it  around  the 
planet.  As  a  result,  the  ionosphere  ends  rather 
abruptly  at  an  altitude  of  only  a  few  hundred 
kilometers.  The  boundary  where  the  iono- 
sphere ends  and  the  region  of  decelerated  solar 
wind  (ionosheath)  begins  is  the  ionopause. 
This  boundary  altitude  is  variable. 

Just  outside  the  ionopause  is  a  large  horizontal 
magnetic  field.  It  contains  some  ionosheath 

plasma  and  some  rapidly  moving  "super- 
thermal"  plasma  of  ionospheric  origin.  This 
large  magnetic  field,  induced  by  the  interac- 
tion of  solar  wind  with  the  ionosphere,  trans- 
mits the  solar  wind  pressure  and  acts  as  a 
"piston"  on  the  ionosphere.  When  the  pres- 
sure is  high,  the  magnetic  field  is  enhanced. 
The  piston  moves  in  and  pushes  the  iono- 
pause to  a  lower  altitude.  When  the  pressure  is 
lower,  the  ionopause  moves  up  (Figure  6-43). 
As  a  result,  the  ionopause  height  is  quite 
variable,  ranging  from  200  km  (124  miles)  to 
over  1000  km  (621  miles)  on  the  dayside.  On 
the  nightside,  there  is  no  direct  interaction  of 
solar  wind  with  the  ionosphere  because  the 
solar  wind  is  deflected  around  the  planet. 
However,  there  must  be  indirect  interactions 
that  we  do  not  yet  fully  understand  because 
even  on  the  nightside  the  height  of  the  iono- 
pause is  usually  less  than  1000  km  (621  miles). 
Also,  there  are  many  variations  over  the 
11 -year  solar  activity  cycle  (Figure  6-44). 

Unlike  the  magnetic  field  just  outside  the 
ionosphere,  the  field  within  it  is  small.  How- 
ever, Orbiter's  magnetometer  detected  unique 
magnetic  structures.  These  structures,  or  flux 
ropes,  are  long,  narrow,  rope-like  regions  of 
strong  magnetic  field  in  which  the  field  lines 
are  twisted.  One  suggestion  is  that  these 
regions  form  from  the  large  magnetic  field 
piled  up  just  outside  the  ionopause.  The  solar 
wind,  "pulling"  on  the  "ends"  of  the  ropes, 
draws  them  down  into  and  through  the  iono- 
sphere. Another  explanation  is  that  magnetic 
flux  ropes  form  in  a  region  of  large  ionospheric 
magnetic  fields  near  the  subsolar  point. 

On  the  nightside,  the  ionosphere's  magnetic 
field  is  most  often  larger  and  more  regular 
than  on  the  dayside.  The  average  field  has  the 
type  of  global  symmetry  expected  from  a 
"draping"  of  solar-wind  field  lines  around 
the  planet. 


Figure  6-42.  The  complex 
environments  of  Venus' 
thermosphere  and  ionosphere 
and  the  planet's  interaction 
with  the  solar  wind.  (Top)  This 
diagram  highlights  major 
discoveries  by  Pioneer  Venus. 
There  is  the  extremely  cold 
nightside  upper  atmosphere, 
gravity  waves  at  predawn  and 
early-dusk  sides,  and  a  dawn 
bulge  in  lighter  constituents  of 
the  atmosphere.  A  large  cloud 
of  atomic  oxygen  extends  over 
the  cold  dayside  thermosphere. 
Low-frequency  radio  bursts 
during  nightside  passages  of 
Orbiter  suggest  lightning 
flashes  in  the  lower  atmo- 
sphere. (Bottom)  This  diagram 
highlights  major  discoveries 
about  the  ionosphere  and 
solar-wind  interaction.  On  the 
sunlit  side  of  Venus,  the  atmo- 
sphere ionizes  to  form  a  dense 
ionosphere.  The  planet  has  no 
intrinsic  magnetic  field.  So  ions 
and  electrons  flow  at  high 
speed  to  the  nightside  and 
form  a  strong  ionosphere 
there.  The  solar  wind  interacts 
with  the  top  of  the  ionosphere 
and  forms  a  bow  shock  that 
moves  in  and  out  from  the 
planet  as  the  strength  of  the 
solar  wind  changes.  There  is 
a  complex  of  plasma  clouds, 
tail  rays,  filaments,  and  iono- 
spheric holes  on  the  planet's 
nightside.  As  a  result  of  the 
Pioneer  Venus  mission, 
scientists  have  examined  the 
ionosphere  of  Venus  in  more 
detail  than  any  other  planet 
besides  Earth. 

Ne  peak 
lonopause  ^-  — 

Ion  outflow 

Dawn  bulge  in 
H,  H+,  He,  D 


Low  dynamic  pressure 

High  dynamic  pressure 

Figure  6-43.  The  two 
sketches  show  the  effects 
of  low  and  high  dynamic 
pressure  of  solar  wind  on 
Venus '  ionosphere  and  the 
changes  in  its  shape  on 
the  nightside. 

Heat  conduction  and  transport  of  electrically 
charged  particles  is  constrained  along  mag- 
netic field  lines  rather  than  at  right  angles  to 
them.  The  flux  ropes  may  affect  electron  and 
ion  temperatures  in  Venus'  ionosphere.  The 
electron  temperature  is  a  few  thousand  kelvin 
on  both  the  dayside  and  nightside  of  the 
planet.  This  is  much  hotter  than  the  neutral 
gas  in  the  thermosphere,  which  has  a  tempera- 
ture of  only  a  few  hundred  kelvin.  Another 
reason  for  high  temperatures  is  that  heat  from 
the  solar  wind  is  "pumped"  into  ionospheric 
electrons  at  the  ionopause.  The  temperature  of 
the  ions  is  also  high,  about  2000  K  on  the 
dayside  and  more  than  4000  K  on  the  night- 
side.  Interactions,  such  as  friction  between  the 
neutral  gas  and  the  ions,  produce  heat  that 
helps  keep  the  ionosphere  hotter  than  the 
neutrals.  On  the  nightside,  some  of  the  energy 
from  rapid  motions  or  horizontal  drifts  of  the 
ions  converts  into  heat  and  makes  the  night- 
side  ions  hotter  than  those  on  the  dayside. 

Orbiter's  ion  mass  spectrometer  established  the 
presence  of  many  different  ions.  From  theo- 
retical studies,  scientists  expected  to  find  O2+, 
O+,  CO2+,  He+,  and  H+  ions.  Other  ions  they 
found  in  Venus'  ionosphere  were  unexpected. 
These  were  C+,  N+,  NO+,  O++,  H2+,  and  N2+. 

Molecular  oxygen  is  the  most  common  ion 
below  200  km  (124  miles)  on  the  dayside  and 
below  160  km  (100  miles)  on  the  nightside. 
Above  an  altitude  of  about  160  to  200  km 
(99  to  124  miles),  atomic  oxygen  becomes  the 
most  common  ion.  In  the  predawn  region  of 
the  nightside,  atomic  hydrogen  ions  are  just  as 
abundant  as  atomic  oxygen  ions. 

There  is  a  strong  day/night  asymmetry,  or  local 
time  variation,  in  the  total  plasma  density. 
Each  ion  species  has  its  own  day/night  asym- 
metry. That  is,  composition  and  total  plasma 
density  depends  on  local  time  (Figure  6-29).  At 
200  km  (124  miles),  atomic  oxygen  ion  con- 
centration gradually  decreases  by  a  factor  of  10 
from  the  dayside  to  the  nightside.  Molecular 
oxygen  ion  density  decreases  rapidly  at  the 
terminator  and  is  almost  one  thousand  times 
less  on  the  nightside  than  on  the  dayside. 
Atomic  hydrogen  and  helium  ions  behave 
quite  differently  from  oxygen  ions  and  are 
greater  on  the  nightside  than  on  the  dayside. 
Yet,  the  nightside  distributions  are  not 
uniform.  There  are  more  hydrogen  than 
helium  ions  in  the  predawn  region,  no  doubt 
reflecting  the  predawn  bulges  in  neutral 
hydrogen  and  helium. 


2.4  Rv 

Solar  min  bow  shock 
Solar  max  bow  shock 


oJ  200 


c  150 





-       2.6 






-  g2.3 




I   I  I 

1975  1977  1979  1981  1983  1985  1987  1989 

Figure  6-44.  The  top  diagram 
shows  how  the  position  of  the 
bow  shock  changes  between 
solar  maximum  and  solar 
minimum.  The  bottom  diagram 
shows  the  position  of  Venus '  bow 
shock  compared  with  sunspot 
numbers  during  the  whole  of  the 
Pioneer  Venus  mission. 


Data  from  Pioneer  Venus  led  to  great  progress 
in  understanding  the  mechanisms  that 
maintained  the  nightside  ionosphere.  The 
problem  is  the  length  of  the  Venusian  night.  It 
is  about  58  Earth  days  and  much  longer  than 
the  lifetime  of  the  ions.  Therefore,  scientists 
did  not  expect  a  significant  ionosphere  of  the 
type  Pioneer  found  on  the  nightside.  Two 
sources  of  ionization  were  identified,  largely 
with  the  help  of  data  from  Orbiter's  instru- 
ments. One  source,  first  supported  by  data 
from  the  Soviet  Venera  spacecraft,  is  the 
bombardment  of  the  nightside  atmosphere  by 
fast  electrons  that  are  energetic  enough  to 
ionize  the  neutral  gas.  This  is  much  like  the 
electron  flux  that  gives  rise  to  terrestrial 
auroras.  The  electrons  originate  in  the  wake  of 
the  planet  outside  the  ionopause.  This 

mechanism  can  account  for  a  large  fraction  of 
the  ionization  in  the  lower  part  of  the  night- 
side  ionosphere.  However,  another  source  of 
ions  is  required  to  account  for  conditions  at 
higher  altitudes  and  to  supplement  the 
"auroral"  source  at  lower  altitudes. 

Instruments  on  Orbiter  detected  large  horizon- 
tal flows,  or  drifts,  of  plasma  from  day  to  night 
hemispheres.  Drift  velocities  were  very  large  at 
high  altitudes  and  near  the  terminator,  up  to 
10  km/sec  (23,000  mph).  Plasma  motions  like 
these  are  more  than  enough  to  maintain  the 
observed  nighttime  ionosphere  at  higher 
altitudes.  A  significant  contribution  also  can 
be  made  to  maintaining  the  lower  ionosphere, 
since  ions,  as  they  flow  to  the  nightside,  also 
sink  to  lower  altitudes.  We  do  not  yet  fully 
understand  the  mechanism  for  the  plasma 
drifts  themselves. 

At  lower  altitudes,  day-to-night  neutral  winds 
help  drag  the  ions  along  to  the  nightside.  At 
very  high  altitudes  near  the  ionopause,  the 
antisunward  flow  of  plasma  on  the  high  side 
of  the  ionopause  can  induce  ionosphere  flow 
below  it.  At  middle  altitudes,  the  day-to-night 
gradients  in  the  ion  densities  seen  by  Orbiter 
can  generate  ion  drifts. 

Concentrations  of  all  ions  show  pronounced 
fluctuations  from  orbit  to  orbit  on  the  night- 
side  as  well  as  near  the  terminators.  Usually 
there  is  an  ordinary-  nightside  ionosphere,  but 
sometimes  the  nightside  ionosphere  disap- 
pears entirely.  Perhaps  solar  wind  (when  its 
pressure  is  large)  sweeps  it  downstream  of 
Venus.  The  nightside  magnetic  field  plays  an 
important  role  in  this.  At  other  times,  the 
nightside  ionosphere  looks  normal  except  for 
localized  holes  in  the  plasma  where  the 
electron  density  is  very  low  and  the  electron 
temperature  is  very  high.  The  magnetic  field  in 
these  holes  aligns  vertically,  indicating  that 

these  holes  may  be  associated  with  the  large- 
scale  structure  of  the  field  on  the  nightside. 
Another  phenomenon  that  is  frequently 
observed  on  night  and  day  hemispheres, 
mostly  near  the  terminators,  is  the  presence  of 
detached  layers  of  clouds  of  ionospheric 
plasma  that  lie  outside  the  ionosphere,  beyond 
the  ionopause.  It  is  likely  that  the  solar  wind, 
or  an  ionosheath  flow,  removes  chunks  of 
plasma  from  the  ionopause  region  and  carries 
this  plasma  downstream  (Figure  6-42). 

The  radio  occultation  experiment  clearly 
showed  a  major  change  in  the  scale  height  of 
the  ionosphere  between  solar  maximum  and 
minimum.  This  may  be  due  to  a  drop  in  exo- 
sphere  temperature  to  about  200  K  at  solar 
minimum.  It  also  could  be  due  to  a  reduction 
in  the  amount  of  atomic  oxygen  in  the  thermo- 
sphere.  This  would  have  the  same  effect  in 
reducing  the  average  scale  height.  Dayside  elec- 
tron density  profiles  at  solar  maximum  in  1980 
and  at  solar  minimum  in  1986  show  the  effects 
of  the  solar  cycle  on  ionospheric  density 
(Figure  6-45).  Scientists  believe  the  depletion  of 
the  upper  ionosphere  at  solar  minimum  is  an 
important  factor  in  the  reduction  of  nightward 
ion  flow,  which,  in  turn,  causes  the  generally 
less  robust  nightside  ionosphere. 

During  the  entry  phase,  Orbiter  investigated 
the  atmosphere  and  ionosphere  under  differ- 
ent conditions  of  solar  activity  from  those 
during  Phase  I.  Researchers  expected  variations 
in  solar  activity  to  have  a  strong  effect  on  the 
nightside  ionosphere.  This  ionosphere  has  two 
sources  of  ionization  at  solar  maximum.  One  is 
ionization  by  electron  precipitation.  The  other 
is  ion  transport  from  dayside  to  nightside. 
Scientists  expected  ion  transport  to  be  reduced 
during  solar  minimum  because  of  a  lower  alti- 
tude of  the  ionopause  at  the  terminator.  This 
would  result  from  reduced  pressure  of  the 
ionospheric  plasma.  At  solar  maximum,  on  the 

other  hand,  solar  wind  has  a  high  dynamic  pres- 
sure. This  also  would  restrict  the  day-to-night 
transport  of  plasma.  It  was  suggested  that  the 
loss  of  ionosphere  at  solar  maximum  arose 
from  a  decreased  transport  of  ions  from  day 
to  night. 

During  Phase  III  of  Pioneer's  mission,  research- 
ers found  that  the  nightside  ionosphere  was 
greatly  reduced  from  conditions  at  solar  max- 
imum. Modeling  ionospheric  processes  sug- 
gested that  some  ion  transport  should  still 
occur  at  that  time.  Since  entry  took  place 
at  a  period  of  intermediate  solar  activity, 
day-to-night  ion  transport  might  still  occur. 
This  agreed  with  observations. 

When  Orbiter  penetrated  below  140  km 
(87  miles)  during  Phase  III,  scientists  had  an 
opportunity  to  study  the  low-altitude  iono- 
sphere with  repeated  sequential  observations 
of  the  nightside  ion  peak.  When  they  com- 
pared the  ion  peak  with  what  they  observed 
during  Phase  I  of  the  mission,  researchers  saw 
interesting  similarities  and  challenging  dif- 
ferences. Earlier  in  the  solar  cycle,  details  of 
the  peak  were  similar  to  those  during  the  final 
encounter.  There  were  no  noticeable  differ- 
ences in  either  the  altitude  of  the  peak  or  the 
maximum  ion  concentrations  at  the  peak. 
However,  the  data  from  Phase  I  showed  a  much 
better  developed  high-altitude  ionosphere, 
with  higher  concentrations  extending  to  higher 
altitudes  than  in  the  final  encounter  data. 
Composition  differences  were  seen  in  the  ion 
mass  spectrometer's  data  between  the  earlier 
mission  and  the  final  encounter.  During  Phase  I, 
the  nightside  ionosphere  at  high  altitudes  was 
more  extensive.  It  had  a  large  concentration  of 
ions  extending  to  high  altitudes.  This  well 
developed  ionosphere  was  maintained, 
researchers  presumed,  by  transport  from  the 
dayside.  As  they  expected,  the  dominant  ion  at 
high  altitudes  was  singly  ionized  atomic 


Figure  6-45.  The  two  curves  on 
this  diagram  show  the  variation 
of  electron  density  with  altitude 
at  two  parts  of  the  solar  activity 
cycle:  1 980,  close  to  maximum, 
and  1 986,  close  to  minimum, 
activity.  The  peak  density 
remains  about  the  same  over  the 
solar  cycle,  but  the  density  in  the 
upper  atmosphere  is  markedly 
reduced  at  solar  minimum. 

800  i- 



•§  400 


Solar  zenith  angle:  60°-70° 
Pioneer  Venus  radio  occultation  experiment 




Average  electron  density 




oxygen.  However,  singly  ionized  hydrogen  was 
severely  depleted  relative  to  the  oxygen  ions. 

Then,  during  the  final  encounter,  concentra- 
tions of  oxygen  ions  were  much  lower  while 
hydrogen  ions  dominated  on  the  nightside  at 
high  altitudes.  Did  the  change  reflect  differ- 
ences in  composition  of  ions  transported  from 
the  dayside  to  the  nightside?  Or  are  hydrogen 
ions  produced  in  some  other  way  and  persist 
when  oxygen  ions  are  not  being  transported 
from  the  dayside?  Researchers  were  not  certain. 

Several  scientists  have  examined  the  day-to- 
night ion  transport  at  low  solar  activity.  By 
combining  analysis  of  data  from  the  ion  mass 
spectrometer  with  mathematical  modeling, 
several  things  were  learned  about  the  impor- 
tance of  plasma  transport  in  the  predawn 
ionosphere,  especially  in  comparison  with 
electron  precipitation.  Scientists  computed  the 
average  peak  density  of  oxygen  ions  as  a 
function  of  solar  zenith  angle.  Then  they 
determined  the  fluxes  of  atomic  ions  or 
precipitating  electrons  needed  to  produce  the 

observed  values.  Calculations  were  compared 
with  observations.  The  comparison  showed 
that  there  must  be  significant  day-to-night 
plasma  transport  at  low  solar  activity.  This 
refuted  earlier  suggestions  that  day-to-night 
transport  would  stop  under  conditions  of  low 
solar  activity.  These  assumptions  had  been 
based  on  a  decreased  solar  flux  leading  to 
dayside  ion  densities  too  low  for  efficient  ion 
transport  to  the  nightside.  However,  calculations 
showed  that  electron  precipitation  cannot 
reproduce  the  observed  helium  ion  densities. 
As  a  result,  researchers  concluded  that  there  are 
significant  day-to-night  fluxes  of  ions,  at  least 
in  the  predawn  bulge  region.  This  occurs  even 
when  solar  activity  is  low. 

Scientists  used  a  one-dimensional  magneto- 
hydrodynamic  model  to  study  the  dayside 
ionosphere.  If  solar  wind  magnetizes  the 
ionosphere  at  solar  minimum,  model  results 
compared  fairly  well  with  observed  electron 
density  profiles.  The  model  also  could 
reproduce  the  layer  of  increased  electron 
density  at  170  to  200  km  (106  to  124  miles) 
that  appeared  in  Orbiter  data.  The  layer 
structure  was  more  apparent  in  the  model 
results  if  it  was  assumed  that  electron  tempera- 
tures below  about  200  km  (124  miles)  are 
much  lower  at  solar  minimum  than  at  solar 
maximum.  Although  there  are  still  uncertain- 
ties about  the  upper  atmosphere  at  solar 
minimum,  the  small  scale  height  of  the 
electron  density  can  be  reproduced  under 
magnetized  conditions.  The  mechanism  for 
structure  formation  is  much  the  same  as  at 
solar  maximum.  Also,  the  ledge  structure  is 
more  apparent  if  low  altitude  electron  tem- 
peratures are  500  K  or  less.  Unfortunately, 
researchers  could  not  determine  electron 
temperatures  in  this  region  from  the  available 
high-altitude  data. 

Orbiter's  discoveries  completely  revolutionized 
thoughts  about  Venus'  ionosphere.  The  region 
turned  out  to  be  much  more  complex  and 
variable  than  expected.  The  ionosphere 
declined  markedly  at  solar  minimum.  Its 
density  also  was  much  lower.  Nightward  ion 
flow  was  greatly  reduced  at  solar  minimum. 
This  resulted  from  a  greatly  reduced  electron 
density  in  the  dayside  upper  ionosphere. 

Solar  activity  varied  greatly  over  Orbiter's 
lifetime.  The  variations  affected  the  properties 
of  the  ionosphere  on  the  planet's  nightside. 
When  solar  ultraviolet  radiation  was  most 
intense  at  solar  maximum,  the  ionosphere 
extended  to  its  highest  level.  Also,  transport  of 
ions  from  the  dayside  was  the  main  source  of 
the  nightside  ionosphere.  By  contrast,  at  solar 
minimum,  nightward  ion  transport  lessened 
and  the  main  source  of  the  nightside  iono- 
sphere appeared  to  be  electron  precipitation. 

In  the  upper  ionosphere  and  the  magnetotail 
near  Venus,  the  effects  of  solar  extreme  ultra- 
violet radiation  are  significant.  This  is  espe- 
cially true  for  the  altitude  profile  of  magnetic 
field,  electron  density,  and  temperature  in  the 
nightside  ionosphere. 

Researchers  discovered  that  electron  density 
decreases  by  about  one  order  of  magnitude 
from  high  to  low  flux  of  solar  extreme  ultra- 
violet radiation.  Also,  the  electron  tempera- 
ture changes  by  a  factor  of  at  least  2.  The 
induced  magnetic  field  also  increases  by  2  to 
3  nT.  In  the  lower  ionospheric  regions  from 
200  to  600  km  (124  to  373  miles),  the  effects 
differ.  At  lower  extreme  ultraviolet  fluxes, 
there  is  a  slightly  reduced  electron  density  and 
a  high  temperature.  These  conclusions  were 
based  on  analysis  of  the  Orbiter's  data  from 
1979  to  1987.  The  results  are  in  accord  with 
entry  phase  observations.  Phase  III  measure- 
ments of  the  electron  density  above  the 



ionospheric  density  peak  were  lower  than 
measurements  at  solar  maximum  during  the 
earlier  parts  of  the  Pioneer  mission. 

Although  the  Orbiter's  measurements  covered 
more  than  a  solar  cycle,  changes  in  altitude  of 
periapsis  created  problems  in  separating  alti- 
tude structure  from  variations  due  to  the  solar 
cycle.  The  evolution  of  the  spacecraft's  orbit 
allowed  study  of  the  main  nightside  ionosphere 
only  during  solar  maximum.  Also,  researchers 
had  to  confine  measurements  of  the  upper 
ionosphere  to  periods  near  solar  minimum. 
The  results  still  show  that  variations  in  solar 
extreme  ultraviolet  radiation  strongly  control 
the  structure  of  the  nightside  ionosphere. 

The  Orbiter's  electron  temperature  probe  made 
important  measurements  during  Phase  III.  The 
median  electron  density  at  the  ionospheric 
peak  at  about  140  km  (87  miles)  altitude  was 
unchanged  from  its  value  at  solar  maximum. 
However,  the  ionosphere  was  increasingly 
depleted  at  high  altitudes.  At  200  km  (124  miles) 
altitude,  the  density  was  reduced  by  a  factor 
of  7.  The  electron  temperature,  by  contrast, 
was  reduced  by  a  factor  of  2  at  140  km 
(87  miles)  and  greatly  enhanced  at  higher  alti- 
tudes. It  even  exceeded  its  value  at  solar  maxi- 
mum. It  was  enhanced  over  the  solar  maximum 
value  by  a  factor  of  1.3  at  200  km  (124  miles) 
and  by  a  factor  of  2  at  500  km  (31 1  miles). 

These  results  generally  supported  earlier  con- 
clusions that  a  reduced  nightward  ion  flow  at 
low  levels  of  solar  activity  depletes  the  night- 
side  upper  ionosphere.  The  lack  of  a  variation 
in  electron  density  near  the  peak  of  electron 
density  over  the  period  between  solar  maxi- 
mum and  the  final  entry  of  Orbiter  led  to 
another  important  conclusion.  Nightward  ion 
transport  is  not  as  important  as  local  ion  pro- 
duction by  energetic  particles  in  forming  the  peak 
density  layer.  The  decrease  in  electron  tempera- 

ture at  low  altitudes  suggests  that  low  densities 
of  the  upper  ionosphere  during  Phase  III  were 
unable  to  support  heat  conduction  from  the 
dayside  ionosphere.  Consequently,  the  lower 
nightside  ionosphere  was  cooled  by  collisions 
with  ions  and  neutrals.  Some  of  its  heat  also 
was  conducted  downward  to  cooler  regions. 

Another  important  feature  of  the  nightside 
ionosphere  is  a  deep  trough  in  electron  den- 
sity. This  trough  typically  appears  between  the 
main  peak  of  the  ionosphere  and  the  upper 
ionosphere.  For  example,  this  trough  was 
observed  on  either  side  of  periapsis  at  an 
altitude  of  about  180  km  (112  miles)  on  two 
consecutive  orbits. 

Also  during  Phase  III,  the  ion  mass- 
spectrometer  data  showed  that  there  were 
lower  numbers  of  all  ion  species  in  the  mid- 
night dusk  sector  than  at  solar  maximum.  The 
most  prominent  change  was  the  decrease  in 
oxygen  ions.  It  was  more  than  one  order  of 
magnitude  from  solar  maximum  to  solar 
minimum.  The  light  hydrogen  ion  is  produced 
in  the  hydrogen  bulge  region  by  charge 
exchange  between  oxygen  ions  and  hydrogen 
transported  from  the  dayside.  Its  concentra- 
tion drops  by  a  factor  of  4. 

Another  interesting  phenomenon  discovered 
by  Orbiter  was  the  disappearing  ionosphere. 
This  occurs  under  solar  maximum  conditions 
when  solar-wind  pressure  increases  beyond 
normal.  It  also  occurs  during  low  solar  activity 
when  dayside  ion  production  falls  to  a  low 
value.  A  disappearing  ionosphere  is  defined  as 
the  state  when  the  ion  density  above  the  main 
ionosphere  peak  becomes  greatly  reduced.  For 
both  cases,  there  is  a  similar  reduction  in  the 
number  density  of  oxygen  ions.  Scientists 
concluded  that  both  reductions  result  from 
decrease  in  the  transport  of  ionization  from 
the  dayside  ionosphere.  More  than  25  of 

these  conditions  were  recorded  during  the 
Pioneer  mission. 

Several  wave  phenomena  were  detected  during 
the  entry  phase  of  the  mission.  The  following 
were  observed:  neutral  density  waves  of  several 
hundred  kilometers  wavelength,  plasma 
density  fluctuations  with  wavelengths  of  about 
one  kilometer,  and  plasma  waves  with  even 
shorter  wavelengths. 

The  kilometer-sized  waves  were  prominent 
during  Phase  III  of  the  mission.  They  were 
often  quasi-sinusoidal  and  occurred  in  a 
relatively  narrow  altitude  layer  of  145  to 
155  km  (90  to  96  miles).  This  area  was  just 
above  the  layer  where  electron  density  peaked. 
Investigators  did  not  observe  these  waves 
above  the  sharp  gradient,  at  the  peak  layer,  or 
below  it.  They  suggested  that  the  waves  are 
generated  by  the  steep  density  gradient 
between  the  main  nightside  ionosphere  from  a 
rapidly  flowing  plasma  above.  There  was  a 
tendency  for  the  waves  to  rise  slightly  to 
higher  altitudes  as  dawn  approached. 

Plasma  waves  were  measured  by  the  electric 
field  detector  throughout  the  low  altitude 
ionosphere  during  the  entry  phase.  The  waves 
fell  into  two  classes.  A  wideband  signal  in 
regions  of  low  magnetic  field  was  restricted  to 
the  5.4  kHz  channel  and  lower.  The  waves  had 
a  roughly  constant  burst  rate  above  an  altitude 
of  160  km  (100  miles)  and  were  attributed  to 
acoustic  mode  waves  generated  by  precipitat- 
ing electrons  from  the  solar  wind.  Whistler 
mode  waves  in  the  100  Hz  channel  were 
attributed  to  lightning.  However,  these  waves 
might  also  result  from  gradient  drift  instabili- 
ties in  a  horizontal  magnetic  field.  Unfortu- 
nately, the  spacecraft  could  measure  only  the 
horizontal  component  of  the  field.  Without  a 
measurement  of  the  radial  field,  it  was  not 
possible  for  scientists  to  resolve  this  question. 

Solar-Wind  Interaction 
The  Sun's  upper  atmosphere,  or  solar  corona, 
is  so  hot  that  it  is  almost  completely  ionized. 
Even  heavy  atoms,  such  as  iron,  have  lost 
many  of  their  electrons.  This  ion-electron  gas 
expands  rapidly  from  the  Sun,  reaching  speeds 
of  over  400  km/sec  (about  1  million  mph),  and 
forms  the  solar  wind.  At  such  speeds,  the  solar 
wind  requires  three  days  to  reach  Venus  and 
four  days  to  reach  Earth.  When  Venus  was 
between  the  Sun  and  Earth,  meteorologists 
used  solar-wind  data  from  Pioneer  Venus  to 
warn  of  impending  solar-wind  disturbances 
on  their  way  to  Earth. 

Interaction  of  the  solar  wind  with  a  planet  is 
similar  to  the  interaction  of  the  atmosphere 
with  a  supersonic  aircraft.  As  an  aircraft  travels 
through  air  at  subsonic  speeds,  pressure  waves 
propagate  ahead  of  the  plane  at  the  speed  of 
sound.  They  warn  of  the  plane's  approach  and 
divert  air  molecules  out  of  its  path.  However, 
when  an  aircraft  travels  at  supersonic  speeds, 
the  warning  cannot  be  transmitted  ahead,  and 
a  shock  wave  forms  in  front  of  the  plane.  This 
shock  diverts  the  air  around  it.  The  solar  wind 
travels  faster  than  the  speed  of  pressure  waves 
that  could  divert  solar-wind  flow  around  a 
planet.  Consequently,  a  shock  wave,  or  bow 
shock,  forms  in  the  solar  wind  in  front  of 
each  planet. 

The  bow  shock  of  Venus  is  in  many  respects 
similar  to  the  bow  shock  of  Earth.  This  might 
be  expected  because  the  properties  of  the  solar 
wind  are  similar  at  Earth  and  at  Venus.  How- 
ever, there  are  differences.  At  Venus,  the 
ionosphere,  which  extends  only  a  few  hundred 
kilometers  above  the  surface,  deflects  the  solar 
wind.  On  Earth,  by  contrast,  the  strong  ter- 
restrial magnetic  field  deflects  the  solar  wind 
at  a  distance  of  over  10  Earth  radii,  tens  of 
thousands  of  kilometers  above  the  planet's 
surface.  This  results  in  a  much  larger  bow 



shock  at  Earth  than  at  Venus.  According  to 
present  models,  the  bow  shock's  distance  from 
the  planet  could  affect  the  energies  of  particles 
the  shock  reflects  back  into  the  solar  wind. 
However,  the  wave  phenomena  at  Venus,  in 
association  with  the  reflected  beams,  seem 
equal  to  the  terrestrial  wave  phenomena  in 
amplitude,  in  frequency  of  occurrence,  and  in 
other  properties. 

Another  way  in  which  Venus  could  differ  from 
Earth  in  its  solar-wind  interaction  is  that  the 
solar  wind  can  reach  Venus'  neutral  atmosphere. 
As  a  result,  processes  that  are  thought  to  be 
important  for  comets  could  occur  at  Venus.  In 
comets,  the  neutral  atmosphere  becomes  ion- 
ized either  by  solar  ultraviolet  radiation  or  by 
the  exchange  of  an  electron  between  a  heavy 
neutral  cometary  ion  and  a  light  solar-wind 
ion,  usually  a  proton.  This  process  adds  mass 
("mass  loads")  to  the  solar  wind  and  slows  it 
down.  Since  the  solar  wind  has  a  magnetic 
field  that  connects  the  slowed  down  solar- 
wind  plasma  to  the  freely  flowing  plasma  far 
from  the  comet,  a  long  magnetic  tail  is  formed 
behind  a  comet,  joining  the  slow  and  fast 
ionized  gas. 

Venus'  neutral  atmosphere  is  bound  to  the 
planet  by  gravity  far  in  excess  of  a  comet's. 
While  Venus'  gravity  can  hold  an  atmosphere, 
the  comet's  cannot.  However,  some  of  the 
neutral  atoms  of  Venus'  atmosphere  do  reach 
the  solar  wind  and  can  be  lost  through  photo- 
ionization  and  charge-exchange  processes. 
There  is  both  direct  and  indirect  evidence  that 
Venus  acts  very  much  like  a  comet  in  its  inter- 
action with  the  solar  wind.  First,  Venus'  bow 
shock  is  slightly  weaker  than  Earth's  shock. 
This  would  occur  if  charge  exchange  behind 
the  shock  led  to  absorption  by  Venus'  atmo- 
sphere. Second,  Venus  has  a  comet-like  mag- 
netic tail.  This  would  occur  if  the  magnetic 

field,  draped  across  the  dayside  of  the  planet, 
became  mass-loaded.  Third,  direct  observa- 
tions have  been  made  of  ions  from  Venus 
flowing  beside  and  behind  the  planet  with  a 
velocity  almost  equal  to  that  of  the  solar  wind. 

The  location  of  the  bow  shock  as  observed  by 
Pioneer  was  somewhat  surprising.  Before  the 
Pioneer  mission,  a  common  belief  was  that 
any  planetary  magnetic  field  of  Venus  would 
be  too  weak  to  hold  off  the  solar  wind.  Hence, 
the  size  of  the  bow  shock  would  be  determined 
by  the  size  of  the  planet  itself  and  would  be 
relatively  unchanging.  However,  Pioneer 
Venus  observed  a  shock  that  is  35%  larger  than 
the  shock  observed  by  the  Soviet  Veneras  9 
and  10  spacecraft.  Why  should  the  size  of  the 
shock  change?  Soviet  measurements  occurred 
at  solar  minimum,  whereas  Pioneer  Venus' 
were  initially  at  solar  maximum.  Scientists 
speculated  that  the  change  in  the  solar  cycle, 
in  particular  in  the  flux  of  ultraviolet  radia- 
tion, caused  changes  in  Venus'  upper  atmo- 
sphere. These  altered  the  rate  of  processes  such 
as  photoionization  and  hence  the  solar-wind 
interaction.  Scientists  investigated  this  specu- 
lation further  during  the  extended  mission  of 
Pioneer  Venus  when  solar  activity  began  to 
decline.  They  confirmed  that  the  bow  shock 
distance  does  change  with  the  solar  cycle 
(Figure  6-44). 

An  electric  field  detector  was  carried  to  Venus 
for  the  first  time  on  Pioneer  Orbiter.  The 
instrument  measured  the  electric  field  associ- 
ated with  oscillations  of  ions  and  electrons.  It 
provided  evidence  for  a  plasma-wave  mecha- 
nism that  couples  the  magnetosheath's  energy 
to  the  ionospheric  plasma  by  whistler  waves. 
It  also  provided  the  basis  for  some  interesting 
and  important  comparisons  among  planetary 
bow  shocks. 

When  scientists  compare  the  plasma  emissions 
at  Venus,  Earth,  Jupiter,  and  Saturn,  they  see 
an  evolution  in  properties.  The  waves  at  Saturn 
are  quite  unlike  those  at  Venus.  The  ratio  of 
solar-wind  velocity  to  the  pressure-wave  veloc- 
ity, or  Mach  number,  determines  the  strength  of 
the  bow  shock.  The  major  change  in  solar 
wind  with  distance  is  that  Mach  number 
increases  with  distance  from  the  Sun.  This 
provides  experimental  verification  that  the 
processes  in  the  shock  change  with  the  shock 
strength.  The  electric  field  detector  also 
provided  evidence  for  lightning  on  the  planet, 
which  confirms  similar  Soviet  observations 
below  the  cloud  tops.  Pioneer  Venus  was  not 
equipped  with  instruments  to  search  visually 
for  lightning,  yet  it  detected  the  electromag- 
netic waves  that  lightning  created.  On  almost 
every  low  altitude  nightside  pass  of  Orbiter,  it 
received  signals  typical  of  those  generated  by 
lightning  discharges. 

As  periapsis  began  to  rise  during  Phase  II, 
researchers  discovered  long  ionospheric  tail 
rays  that  extended  more  than  3000  km 
(1870  miles)  downstream.  This  is  called  the 
ionotail.  The  tail  rays  are  thought  to  be 
ionospheric  hydrogen  and  oxygen  ions  accel- 
erated to  velocities  high  enough  for  them  to 
escape  the  planet.  Their  discovery  is  important 
to  studies  of  how  the  water  of  ancient  Venus' 
oceans  might  have  been  scavenged  from  the 
atmosphere  over  geological  time. 

The  plasma  analyzer  made  measurements  of 
conditions  in  the  ionosheath  downstream  of 
the  planet  during  Phase  III.  Researchers  found 
a  depletion  of  energetic  ionosheath  electrons 
downstream  from  the  terminator,  similar  to 
that  in  the  Mariner  10  data.  There  are  several 
explanations  for  this  condition.  If  the  deple- 
tion is  due  to  atmospheric  scattering,  there 
would  be  electrons  traveling  along  draped 
magnetic  flux  tubes  threading  through  Venus' 

neutral  atmosphere.  These  electrons  would 
lose  energy  from  impact  ionization  with  oxy- 
gen. Atmospheric  loss  could  provide  a  natural 
process  for  electrons  at  energies  of  about 
100  eV  to  be  selectively  removed.  Energetic 
electron  depletion  might  alternatively  be  a 
strong  draping  that  connects  the  depletion 
region  magnetically  to  the  weak  downstream 
bow  shock.  This  connection  could  reduce  the 
electron  source  strength.  It  is  not  clear  from 
the  data  whether  the  energetic  electron 
depletions  observed  by  Mariner  10  and  Pioneer 
Venus  Orbiter  result  from  depletion  by 
atmospheric  scattering  or  from  a  reduced 
source  strength. 

The  Exosphere 

The  exosphere  forms  the  outermost  fringe  of 
the  atmosphere.  In  this  region,  atoms  move 
in  ballistic  trajectories  and  rarely  collide  with 
each  other.  Orbiter's  ion  mass  spectrometer 
discovered  that  the  number  of  hydrogen  atoms 
increased  steadily  through  the  night,  then 
decreased  quickly  through  the  day.  The  atoms 
were  effectively  "trapped"  by  the  very  low  tem- 
perature of  the  nightside  exosphere.  Hydrogen 
atoms  were  so  scarce  in  the  dayside  exosphere 
that  oxygen  replaced  them  in  dominance.  The 
oxygen  atoms  are  unusual  in  that  they  are  very 
hot.  They  are  produced  by  decomposition  of 
ionized  oxygen  molecules.  This  process  at 
lower  altitudes  is  the  mechanism  by  which 
ultraviolet  sunlight  heats  the  atmosphere. 

Hydrogen  in  the  exosphere,  as  identified  from 
Lyman  alpha  glows,  showed  two  components. 
At  lower  altitudes,  there  was  a  component  of 
the  exosphere  at  275  K.  However,  this  com- 
ponent was  negligible  above  3000  km 
(1860  miles)  and  allowed  Orbiter  to  detect  a 
nonthermal  component.  There  is  now  general 
agreement  that  various  reactions  drawing  on 
the  energy  of  the  ionosphere  produce  this 
nonthermal  component  of  the  exosphere. 


In  the  exosphere,  collisions  with  ambient  gases 
do  not  slow  the  quickly  moving  atoms  of  oxy- 
gen and  hydrogen.  They  rise  thousands  of 
miles  into  space  and  form  the  first  obstacle  to 
the  solar  wind  approaching  Venus.  Some  of 
these  atoms  attain  velocities  high  enough  to 
escape  into  space. 

Hydrogen  and  oxygen  atoms  in  the  exosphere 
escape  by  different  processes.  On  the  dayside, 
extreme  ultraviolet  sunlight  ionizes  oxygen 
atoms,  then  the  solar  wind  carries  them  away. 
On  the  nightside,  many  more  hydrogen  atoms 
have  charge-exchange  collisions  with  hot  pro- 
tons. The  hot  protons  capture  electrons  and 
become  fast  moving  hydrogen  atoms  that  pos- 
sess enough  velocity  to  escape  Venus'  gravity. 
Cool  hydrogen  atoms  lose  an  electron  and 
become  cool  protons.  These  move  too  slowly 
to  escape.  They  remain  in  the  exosphere. 

The  Intrinsic  Magnetic  Field 

Except  for  Venus  and  the  Moon,  and  possibly 
Mars,  every  planet  visited  by  spacecraft  has  a 
magnetic  field  that  is  thought  to  be  internally 
driven.  Some  scientists  speculated  before 
Pioneer  reached  Venus  that  perhaps  the  planet 
had  an  internal  magnetic  field  too  weak  for 
previous  missions  to  detect.  However,  Orbiter 
probed  thoroughly  for  a  field  with  highly 
sensitive  instruments  and  found  none. 

During  Phase  III,  the  magnetometer  made 
repeated  measurements  from  midnight  to 
about  4:30  a.m.  at  altitudes  below  185  km 
(115  miles).  Data  from  this  phase  were  from 
the  bottom  of  the  nightside  ionosphere.  They 
were  important  because  researchers  wanted 
to  obtain  information  about  the  possibility  of 
an  internal  planetary  field  contributing  to  the 
observed  magnetic  field.  In  this  region, 
explored  by  Orbiter  during  Phase  III,  it  was 
found  that  the  magnetic  field  was  generally 
stronger  at  comparable  altitudes  than  it  was  at 

times  of  high  solar  activity.  Also,  at  solar 
minimum,  this  increase,  coupled  with  a 
decrease  in  electron  density,  caused  the  ratio 
of  the  magnetic  pressure  to  the  thermal 
pressure  to  approach  unity  at  this  altitude.  At 
solar  maximum,  however,  the  ratio  was  much 
less  than  unity. 

Researchers  observed  another  major  difference 
between  conditions  at  the  start  of  the  mission 
and  at  the  entry  phase.  From  160  to  200  km 
(100  to  125  miles),  the  magnetic  field  pressure 
exceeded  that  of  the  ionospheric  plasma.  How- 
ever, below  150  km  (93  miles),  the  induced 
field  was  weaker,  diminishing  sharply.  This 
permitted  researchers  to  search  for  an  intrinsic 
field  during  Phase  III.  Pioneer  measurements 
clearly  show  that  Venus'  intrinsic  magnetic 
field  is  extremely  weak.  The  data  showed  no 
evidence  of  a  planetary  field.  Venus  has  the 
lowest  magnetic  moment  of  any  planet  visited 
by  spacecraft  so  far.  This  field  is  so  weak  that  it 
can  play  no  role  in  the  interaction  of  Venus 
with  the  solar  wind. 

One  of  the  principal  unsolved  problems  of 
geophysics  is  the  nature  of  the  source  of  the 
terrestrial  dynamo  that  generates  the  magnetic 
fields  of  Earth  and  the  other  planets.  Scientists 
hoped  that  a  measurement  of  a  magnetic  field 
of  Venus,  a  planet  which  appears  in  many 
respects  to  be  Earth's  twin,  would  help  clarify 
the  effect  of  spin  rate  on  the  dynamo  process. 
Venus  spins  on  its  axis  much  more  slowly  than 
does  Earth,  once  in  243  Earth  days.  Dynamo 
theories  predict  that  a  planetary  dynamo,  such 
as  that  generating  Earth's  field,  should  depend 
on  spin  rate.  If  Venus'  dynamo  were  identical 
to  Earth's,  but  weaker  in  proportion  to  the 
spin  rate,  the  planet  would  have  a  magnetic 
field  that  could  easily  be  detected.  However,  it 
does  not,  so  other  explanations  are  needed. 

A  planetary  magnetic  dynamo  requires  a 
highly  electrical  conducting  liquid  core.  The 
absence  of  a  conducting  core  may  explain  why 
Earth's  satellite,  the  Moon,  does  not  have  a 
magnetic  field.  Unfortunately,  it  does  not 
explain  the  absence  of  a  field  of  Venus.  Under 
the  temperatures  and  pressures  in  the  core  of 
Venus,  there  should  be  a  highly  conducting 
fluid.  However,  the  composition  and  electrical 
conductivity  of  the  fluid  may  be  different  from 
those  of  Earth.  Although  Venus  appears  to  be 
Earth's  twin  in  size,  it  may  not  be  a  twin  in 
chemical  composition  since  it  formed  at  a 
different  place  in  the  solar  nebula  and  prob- 
ably at  a  different  temperature. 

Another  possible  difference  is  the  weakness  of 
any  energy  source  which  would  drive  Venus' 
dynamo.  Present  thinking  about  our  planet's 
dynamo  is  that  a  solid  inner  core  is  growing  at 
the  center  of  the  Earth.  As  this  core  grows,  it 
releases  its  latent  heat  of  fusion  into  the  sur- 
rounding fluid.  Scientists  calculate  that  this 
energy  source  is  stronger  than  the  once  pop- 
ular radioactive  heating  mechanism.  Pressure 
and  temperature  at  the  core  of  Venus  are  only 
slightly  less  than  at  Earth's  core.  However,  this 
difference  may  be  sufficient  to  prevent  solid- 
ification of  Venus'  inner  core.  This  could  be 
true  even  if  the  internal  composition  of  the 
two  planets  are  the  same. 

Lack  of  a  magnetic  dynamo  on  Venus  today 
has  implications  for  Earth.  Suppose  the  reason 
for  lack  of  a  dynamo  is  that  Venus  has  a  totally 
liquid  core.  Then  Earth  may  not  have  had  a 
magnetic  dynamo  and  an  intrinsic  magnetic 
field  until  its  solid  inner  core  began  to  form. 
Today,  Earth  is  protected  by  a  strong  magnetic 
field  that  isolates  its  atmosphere  from  the  solar 
wind.  As  a  result,  there  is  very  little  loss  of 
Earth's  atmosphere  to  the  solar  wind.  If  the 
Earth  did  not  always  have  a  strong  magnetic 
field,  there  would  have  been  times  when  it  was 

not  protected.  Known  magnetic  reversals  of 
Earth's  field  also  would  have  led  to  periods 
when  our  planet  was  not  protected  from  the 
solar  wind.  These  effects  would  have  to  be 
considered  in  determining  how  our  planet 
evolved  so  that  life  could  originate  and 
develop  on  it. 

If  we  gain  a  better  understanding  of  the 
terrestrial  dynamo  process,  scientists  may  be 
able  to  infer  some  of  the  internal  properties  of 
the  planet.  On  the  other  hand,  if  some  of 
these  internal  properties  become  known 
through  other  means,  they  may  be  able  to  use 
the  absence  of  a  magnetic  field  of  Venus  to 
help  understand  the  dynamo  process.  In  short, 
all  that  can  be  unambiguously  stated  is  that 
Venus  at  present  does  not  have  a  magnetic 
dynamo.  The  nature  of  the  source  of  planetary 
magnetic  fields  still  remains  one  of  the  major 
unsolved  problems  of  geophysics. 

Interplanetary  Magnetic  Field 

The  interplanetary  magnetic  field  originates 
from  the  solar  dynamo,  which  generates  a 
magnetic  field  on  the  Sun.  The  field  is  borne 
outward  by  the  solar  wind  and  varies  with 
conditions  on  the  solar  surface  that  produce 
the  solar  wind.  Researchers  combined  magne- 
tometer data  from  Orbiter  at  0.7  AU  (astro- 
nomical unit)  from  the  Sun  with  similar  data 
from  the  IMP-8  spacecraft  at  1.0  AU.  They  com- 
pared the  long-term  behavior  of  the  interplan- 
etary magnetic  field  over  a  solar  cycle  at  these 
two  locations.  They  discovered  that  at  Venus 
there  was  an  enhancement  of  the  typical  field 
magnitude  during  declining  solar  activity  com- 
pared with  the  field  at  maximum  or  minimum 
solar  activity.  This  is  different  from  fields  in 
the  vicinity  of  Earth.  Here,  we  observe  high 
fields  most  frequently  during  solar  maximum 
conditions.  This  suggests  that  the  intensity  of 
fields  from  transient  solar  disturbances,  such 
as  coronal  mass  ejections,  depends  upon 


Figure  6-46.  The  orbits  of  Earth, 
Venus,  and  Comet  Halley.  The 
comet  passed  perihelion  above 
Venus  in  February  7  986.  Orbiter 
then  had  a  unique  opportunity 
to  observe  the  cometary  activity 
at  the  important  period  of 
perihelion  passage  when  the 
comet  was  closest  to  the  Sun 
and  most  active. 


Orbit  of  Comet  Halley 






position  within  the  heliosphere.  However, 
these  results  apply  only  to  the  solar  cycle  that 
Orbiter  observed.  We  need  more  data  to  extend 
the  results  into  a  general  rule  of  interplanetary 
field  intensity.  Scientists  do  not  yet  have  a 
clear  explanation  of  why  the  strong  transients 
they  detected  at  Venus  peaked  in  the  declining 
phase  of  solar  activity. 

The  observations  from  Pioneer  were  significant 
because  data  on  the  strength,  orientation,  and 
variability  of  the  interplanetary  magnetic  field 
are  required  for  studies  of  how  the  solar  wind 
interacts  with  planets  and  comets.  The 
magnitude  of  the  interplanetary  field  affects 
the  structure  and  shape  of  bow  shocks  at  the 
planets.  Orientation  of  the  magnetic  field 
determines  the  efficiency  of  solar  wind 
coupling  with  strongly  magnetized  planets 
and  the  mode  of  ion  pickup  from  the  iono- 
spheres of  weakly  magnetized  planets  and 
from  comets. 

Comets  Observed  by  Orbiter 

Comet  Halley  passed  within  40  million  km 
(25  million  miles)  of  Venus  only  5  days  before 
the  comet's  perihelion  on  February  9,  1986.  At 
perihelion,  the  comet  was  87.9  million  km 
(54.6  million  miles)  from  the  Sun.  Orbiter, 
at  the  time,  was  close  to  40.2  million  km 
(25  million  miles)  from  the  comet  (Figure  6-46). 

The  Science  Steering  Group  agreed  to  forego 
normal  Orbiter  observations  of  Venus  for 
70  days  from  late  December  1985  to  early 
March  1986.  They  devoted  the  spacecraft's 
resources  instead  to  observations  of  the  comet 
with  the  ultraviolet  spectrometer.  Each  day, 
mission  controllers  maneuvered  the  spacecraft 
so  that  the  spectrometer  could  observe  the 
region  near  the  comet's  nucleus.  At  the  same 
time,  the  solar  panels  had  to  gather  sunlight 
and  the  antenna  had  to  point  toward  Earth. 
More  than  40  maneuvers,  though  complicated, 
were  highly  successful.  The  only  losses  of  data 
were  during  superior  conjunction  in  January 
1986.  At  this  time,  Venus  and  Orbiter  were  on 
the  far  side  of  the  Sun  from  Earth.  A  solar  flare 
also  interrupted  radio  communications  on 
February  3. 

Researchers  obtained  data  in  near  real  time  for 
ultraviolet  emissions  from  hydrogen,  oxygen, 
carbon,  and  hydroxyl  radicals  in  the  comet's 
coma.  From  these  data,  they  calculated  the 
production  rates  of  water  and  carbon-bearing 
ices  from  the  nucleus.  The  production  rate  of 
water  rose  from  10  tonnes  (approximately 
1.1  U.S.  tons)  per  second  at  1  AU  inbound  to 
50  tonnes  per  second  shortly  after  perihelion. 
Then  it  fell  slowly  to  40  tonnes  per  second  at 
the  time  when  the  Soviet  Vega  1  spacecraft 

Figure  6-47.  Orbiter  obtained 
valuable  data  about  the  release 
rate  of  materials  from  the 
comet's  nucleus  during  its 
perihelion  passage. 

encountered  the  comet  on  March  6.  (See 
Chapter  7  for  results  of  Vega's  encounter.) 

After  perihelion,  the  water  production  rate 
varied  with  a  complex  7.4-day  periodicity. 
These  results  provided  a  unique  description  of 
the  comet's  behavior  during  the  otherwise 
poorly  observed  perihelion  passage.  Other 
spacecraft  could  observe  the  comet  on  its 
inbound  and  outbound  paths  only.  The  results 
from  Orbiter  coupled  with  those  from  other 
sources  (Interplanetary  Ultraviolet  Explorer 
(IUE))  showed  that  Halley  lost  about 
270  million  tonnes  of  water  during  its  perihe- 
lion passage.  If  the  comet's  density  is  0.3,  the 
loss  of  water  would  amount  to  about  10  meters 
of  material  from  active  areas  of  the  nucleus 
(Figure  6-47). 

From  February  2  to  6,  1986,  a  special  series  of 
operations  allowed  Orbiter's  ultraviolet  spec- 
trometer to  acquire  a  spin-scan  image  of 
Halley's  entire  hydrogen  coma  (Figure  6-48). 
At  that  time,  the  coma  was  about  25  million 
km  (15  million  miles)  across.  The  image  clearly 
showed  the  effects  of  solar  radiation  pressure 
on  the  trajectory  of  cometary  hydrogen.  It  also 
showed  the  signature  of  the  different  hydro- 
gen atom  velocities  associated  with  the  two 
main  production  processes:  photodissociation 
of  water  molecules  and  of  hydroxyl  radicals. 

In  addition  to  Comet  Halley,  Orbiter  observed 
six  other  comets.  Among  these,  it  observed 
Comet  Encke  at  0.58  AU  outbound.  Investigators 
deduced  Encke's  water  production  rates  from 
measurements  of  atomic  hydrogen.  The  comet 


Figure  6-48.  The  spacecraft 
imaged  the  coma  and  hydrogen 
halo  of  the  comet.  Scientists 
determined  the  comet's  gas 
composition,  rate  of  water  loss 
from  the  nucleus,  and  ratio  of 
dust  to  gas  in  the  coma  and  in 
the  nucleus. 


was  losing  water  faster  than  anyone  expected. 
It  appears  from  the  data  that  water  ice  and 
dust  are  distributed  unevenly  in  the  nucleus' 
cometary  material. 

When  combined  with  data  from  IUE,  the 
Orbiter  data  demonstrated  a  profound  and 
unexpected  difference  between  the  comets' 
visual  and  ultraviolet  light  curves.  Activities 
of  other  comets  observed  from  Orbiter 
(Giacobini-Zinner  in  September  1985,  Wilson 
in  March-April  1987,  Nishikawa-Takamizawa- 
Tago  in  April  1987,  McNaught  in  November 
1987,  and  Machholz  in  September  1988)  fell 
within  a  factor  of  2  of  those  scientists  expected. 
Comparison  of  carbon/hydrogen  ratios  for 
Nishikawa-Takamizawa-Tago  with  those  for 

Halley  and  Wilson  led  to  a  prediction  that 
Nishikawa-Takamizawa-Tago,  like  Halley,  but 
unlike  Wilson,  is  a  periodic  comet.  Scientists 
subsequently  confirmed  this  prediction  when 
they  calculated  the  comet's  orbit  more  pre- 
cisely as  an  ellipse. 

'*  • 

Observations  of  Giacobini-Zinner  from  Pioneer 
Orbiter  coincided  with  the  passage  of  the 
International  Cometary  Explorer  (ICE)  space- 
craft through  the  comet's  tail.  The  comet  had 
just  passed  perihelion  and  was  about 
160  million  km  (100  million  miles)  from  the 
spacecraft.  Also,  the  Pioneer  observations 
showed  that  Giacobini-Zinner  is  much  more 
active  than  Encke,  but  less  active  than  Halley. 
Lyman  alpha  ultraviolet  emissions  from  the 

hydrogen  corona  of  Giacobini-Zinner  were 
detected  on  either  side  of  the  nucleus  as  far  as 
5  million  km  (3  million  miles)  from  it. 

Oceans  on  Venus? 

There  have  long  been  speculations  that  early 
in  its  history  Venus  had  a  temperate  climate 
and  possessed  oceans  like  Earth's.  These  oceans 
vaporized  as  Venus  grew  hotter.  A  runaway 
greenhouse  effect  some  three  billion  years  ago 
resulted  as  a  cool  early  Sun  increased  its 
luminosity.  The  oceans  evaporated,  and  solar 
ultraviolet  radiation  split  the  water  molecules 
into  oxygen  and  hydrogen.  The  lightweight 
hydrogen  atoms  easily  attained  escape  velocity 
from  the  planet  and  sped  off  into  space.  The 
discovery  by  Pioneer  that  heavy  hydrogen 
(deuterium)  is  150  times  more  plentiful  on 
Venus  than  on  Earth  has  been  taken  as 
evidence  that  Venus  once  had  150  times  as 
much  water  in  its  atmosphere  as  today.  The 
heavier  deuterium  could  not  reach  escape 
velocity  as  readily  as  ordinary  hydrogen.  This 
suggests  there  was  enough  water  on  Venus  to 
cover  its  surface  to  a  depth  of  several  feet. 

When  Orbiter  made  its  final  descent  into  Venus' 
atmosphere,  it  found  evidence  of  3.5  times  as 
much  water  as  that  from  the  hydrogen/ 
deuterium  ratio.  Investigators  discovered  an 
unexpected  escape  mechanism  capable  of 
accelerating  both  hydrogen  and  deuterium 
from  the  planet.  A  lot  more  hydrogen  must 
have  escaped  than  previously  thought.  In  turn, 
this  means  that  there  must  have  been  more 
water  on  early  Venus.  Other  theorists  suggested 
that  conditions  on  an  early  Venus  might  have 
developed  an  almost  explosive  pouring  of 
hydrogen  into  space.  That  process  would  have 
carried  along  many  deuterium  atoms,  too.  If 
such  a  process  did  occur,  deep  oceans  like 
those  on  Earth  could  have  been  lost  to  Venus 
in  only  a  few  hundred  million  years. 

There  are  other  reasons  cited  for  why  Venus 
should  have  possessed  early  oceans.  All  the 
terrestrial  planets  are  thought  to  have  formed 
from  a  mix  of  planetesimals  moving  around 
the  Sun  in  fairly  eccentric  orbits.  As  they  grew 
from  these  planetesimals,  all  the  planets  would 
have  received  similar  proportions  of  volatiles. 
Also,  the  planets  should  have  received  similar 
amounts  of  volatiles  from  cometary  impacts. 
Since  Venus  has  about  the  same  abundances  of 
at  least  two  other  volatiles,  nitrogen  and  carbon, 
it  should  have  had  the  same  abundance 
of  water. 

However  tantalizing  speculative  theories  may 
be  that  Venus  once  had  terrestrial-type  oceans, 
we  need  much  more  intensive  analysis  of 
available  data  and  new  missions  to  Venus  to 
resolve  the  uncertainty. 


Summary  of  Major  Results  from 
Pioneer  Venus 

The  following  text  highlights  Pioneer's 
findings  about  Venus  or  confirms  earlier 
observations.  During  the  mission,  Pioneer 

•  Obtained  radar  altimetry  for  nearly  all  the 
surface  of  the  planet  and  many  radar  images; 
discovered  volcanic  and  tectonic  features 
such  as  rift  valleys,  mountains,  continents, 
and  volcanoes.  Found  that  there  is  a 
unimodal  distribution  of  topography  (quite 
unlike  the  bimodal  distribution  on  Earth) 
and  a  dearth  of  elevated  regions  of  continen- 
tal size.  Confirmed  the  existence  of  great 
troughs  (rift  valleys);  however,  researchers 
found  no  evidence  for  continuous  ridge  sys- 
tems that  are  typical  of  the  terrestrial  plate 
tectonics  system. 

•  Obtained  measurements  of  the  gravity  field. 
When  combined  with  radar  altimetry 
results,  this  measurement  showed  that  the 
interior  behavior  of  Venus  is  more  like  that 
of  Earth  than  Mars  or  the  Moon.  However, 
there  is  a  great  difference  between  Venus 
and  Earth.  On  Venus,  there  is  a  strong 
positive  correlation  of  gravity  with  topogra- 
phy at  all  wavelengths. 

•  Determined  the  structure  of  the  clouds 
globally  and  vertically — their  layers,  distribu- 
tion of  different  sized  particles,  composition, 
and  optical  properties — confirming  results 
from  earlier  Soviet  probes. 

•  Made  refined  measurements  of  composition 
and  abundances  of  major,  minor,  and  noble 
gas  species  in  the  lower,  mixed  atmosphere 
and  in  the  upper,  diffusively  separated 

•  Discovered  much  structure  in  the  polar 
regions  of  the  atmosphere,  thereby  clarifying 
our  understanding  of  the  circulation  pattern 
in  those  regions. 

•  Discovered  that  sulfur  dioxide  is  an  impor- 
tant absorber  of  ultraviolet  radiation  at 
wavelengths  below  3200  angstroms,  but 
that  another  absorber  must  be  present  to 
account  for  absorption  at  longer  wavelengths. 

•  Detected  radio  signals  that  some  researchers 
believe  originate  from  lightning  discharges 
in  the  clouds  of  Venus,  thereby  confirming 
some  observations  the  Venera  probes  made. 

•  Obtained  much  new  data  about  atmospheric 
state  properties  (temperature,  pressure, 
density)  globally  and  vertically  from  the 
surface  through  the  clouds  and  into  the 
upper  atmosphere. 

•  Obtained  measurements  of  vertical  profiles 
of  wind  velocities  at  four  probe  locations  and 
global  wind  measurements  at  the  cloud  tops. 

•  Determined  the  sinks  for  solar  radiation  and 
the  sources  and  sinks  of  infrared  radiation  in 
the  lower  atmosphere  and  clouds  at  four 
locations  characterizing  daytime,  nighttime, 
low  latitude,  and  high  latitude  conditions. 

•  Discovered  that  the  high  atmosphere  well 
above  the  cloud  tops  is  much  colder  at  night 
than  in  the  daytime. 

•  Combined  these  observations  into  a  con- 
ceptual general  meteorological  model  for 
comparative  meteorological  studies. 

•  Mapped  the  airglow  on  the  dark  side 
of  Venus. 

•  Provided  strong  support  for  a  greenhouse 
effect  that,  coupled  with  global  dynamics, 
explains  the  high  surface  temperature. 

•  Determined  the  global  properties  of  the 
ionosphere — its  ion  composition,  tempera- 
ture, flows,  electron  concentration  and 
temperature,  modification  of  ionospheric 
properties  by  input  from  the  solar  wind,  and 
the  production  and  maintenance  of  a 
nightside  ionosphere. 

•  Determined  the  nature  of  the  solar-wind 
interaction  with  the  planet.  This  included 
temporal  and  spatial  studies  of  the  location 
of  the  bow  shock  and  ionopause  and  of 
particle  and  energy  input  to  the  atmosphere 
over  a  complete  solar  cycle. 

•  Confirmed  that  Venus  has  little  if  any 
intrinsic  magnetic  field,  and  set  a  very  low 
upper  limit  on  a  magnetic  moment  of 
the  planet. 

•  Determined  how  the  ionosphere  varies  over 
the  11 -year  cycle  of  solar  activity. 

•  Determined  how  the  solar  activity  cycle 
affects  the  atmosphere  in  general  and  the 
interaction  of  the  planet  with  the  solar  wind. 

•  Made  important  discoveries  about  the  rate  of 
evolution  of  materials  from  cometary  nuclei. 


•  Worked  in  cooperation  with  other  spacecraft  

missions  to  map  the  positions  of  over 
30  gamma-ray  burst  events. 



R.  Z.  Sagdeev,  V.  I.  Moroz,  and  T.  Breus 

Venus,  the  planet  nearest  Earth,  has  always 
been  of  interest  to  the  Soviet  Space  Program — 
it  has  sent  the  largest  number  of  unmanned 
space  probes  there.  The  planet's  many  features 
that  are  similar  to  our  own  Earth  has 
prompted  this  keen  interest  in  Venus.  The  two 
planets'  mass  and  geometry  are  indeed  similar, 
and  they  receive  roughly  equal  energy  from 
the  Sun. 

Some  20  years  ago,  scientists  thought  that 
Earth's  "sister  planet"  was  its  exact  replica. 

They  envisioned  it  with  a  slightly  warmer 
|^       surface,  hydrosphere,  and,  possibly, 
biosphere.  Yet,  as  the  first  studies 
revealed,  there  are  drastic 
differences  in  climate.  The 
temperature  on  the 
Venusian  surface  averages 
735  K  (about  462°C,  or 
864°F).  However,  the  average 
temperature  of  Earth's  surface  is 
15°C  (59°F).  Furthermore,  Venus' 
entire  surface,  regardless  of  latitude  or 
time  of  day,  seems  to  be  uniformly 
heated.  This  situation  is  distinctly  different 
from  conditions  on  Earth. 

All  these  unique  features  of  the  Venusian 
atmosphere,  however,  have  been  established 
only  in  the  era  of  space  exploration. 

Soviet  Spacecraft 

In  the  second  half  of  the  1950s,  radio  tele- 
scopes yielded  data  about  the  high  tempera- 
ture of  Venus'  surface.  So  unexpected  was  this 

information,  not  all  scientists  believed  it.  To 
settle  the  issue,  the  first  Soviet  interplanetary 
automatic  stations  to  Venus  had  "surface 
phase  state"  sensors  onboard.  These  sensors 
could  determine  whether  the  vehicle  had 
landed  on  a  solid  surface  or  if  ocean  waves 
were  rocking  it. 

On  October  18,  1967,  Venera  4,  the  first 
spacecraft  to  descend  into  Venus'  atmosphere 
with  a  parachute,  had  no  such  sensor  onboard. 
However,  for  this  mission,  the  spacecraft  had 
protection  against  the  extremely  high  tem- 
peratures it  encountered.  This  protection 
allowed  it  to  take  actual  measurements  of  the 
conditions  it  faced.  Subsequent  Venera 
spacecraft — Venera  5/Venera  6  (1969)  and 
Venera  7/Venera  8  (1972)— added  to  the 
information  (see  Table  7-1).  These  probes 
yielded  detailed  information  about  variations 
in  temperature,  pressure,  and  density  of  the 
Venusian  atmosphere  with  altitude.  Venera  7 
and  Venera  8  made  soft  landings  and  transmit- 
ted signals  directly  from  the  planet's  hot 
surface.  Instruments  aboard  Venera  8  took  the 
first  scattered  solar  radiation  measurements. 
They  also  furnished  information  about  soil 
composition,  including  uranium,  potassium, 
and  thorium. 

Some  years  before  NASA 
published  the  first  edition 
of  this  book  in  1983,  Soviet 
space  scientists  graciously 
contributed  this  chapter.  In 
it,  they  detailed  their  Venera 
missions  4  through  12 
(1967-1978).  They  also 
mentioned  the  "upcoming" 
(1984)  Vega  project  at  the 
end  of  their  text.  To  bring 
events  up  to  date,  our 
American  authors  have 
returned  and  added  their 
own  text  (1994)  at  the 
chapter's  end.  They  describe 
the  flights  of  the  Soviet 
Veneras  13  through  16 
(1981-1982).  They  also 
give  results  of  Vegas  1  and 
2,  including  the  successful 
Comet  Halley  ftyby  in  1986. 


Table  7-1.  Soviet  Space  Vehicles  That  Studied  Venus,  1967  to  1978 

Space  vehicle 


Landing  site 








Solar  angle, 

Venera  4 

module  + 
flyby  vehicle 






Descent  module:  Temperature,  pressure, 
density,  wind  velocity;  CC>2,  N2,  H2O 
content  at  altitudes  of  55  to  25  km;  ion 
number  density  in  the  ionosphere, 
magnetic  field 
Flyby  vehicle:   1  1  a  —  and  01/1  300A  - 
radiation  in  upper  atmosphere;  ion  flux  in 
region  of  solar-wind  flow  around  planet; 
magnetic  field 

Venera  5 

Same  as 


5/1  6/69 




Temperature,  pressure,  wind  velocity,  CO2, 
N2,  H2O  content  at  altitudes  of  55  to 

Venera  6 

Same  as 


5/1  7/69 




Same  plasma  measurements  as  on  Venera  4 

Venera  7 

module  (soft 

7/1  7/70 






Venera  8 

Same  as 






Temperature,  pressure,  solar  scattered 
radiation  (from  55  km  to  surface),  wind 

Venera  9 

module  (soft 
landing)  + 






Descent  module:  Temperature,  pressure, 
wind  velocity;  CC>2,  N2,  H2O  content,  solar 
scattered  radiation  (several  filters),  clouds 
(nephelometer),  panoramic  survey  of 
Satellite:  TV  survey  of  clouds,  IR  radiometry, 
spectroscopy  of  the  day-  and  nightside; 
photopolarimetry;  energy  spectra  of  ions 
and  electrons,  electron  and  ion  number 
densities  and  temperatures,  magnetic  field 
in  region  of  solar-wind  interaction  with 
planet;  radio  occultations 

Venera  1  0 

Same  as 






Same  as  above 

Venera  1  1 

module  (soft 
landing)  + 
flyby  vehicle 






Descent  module:  Temperature,  pressure, 
wind  velocity,  composition  (mass 
spectrometer);  solar  scattered  radiation 
spectrum;  nephelometer,  thunderstorm 
Flyby  vehicle:  Upper-atmosphere  UV 

Venera  1  2 

Same  as 






Same  as  above;  gas  chromatograph  and 
measurements  of  particle-composition  of 
cloud  layer 


aMinus  sign  denotes  night  landing  (the  Sun  below  the  horizon).  First  generation  vehicles  landed  at  night  (except  Venera  8,  which 
landed  near  the  terminator).  It  was  necessary  since  information  was  transmitted  directly  to  Earth.  Since  Venera  9  information  was 
relayed  via  the  artificial  satellite  from  the  lander  and  the  landing  was  made  during  the  day,  this  was  widely  used  to  study  solar 
radiation  propagation  in  the  atmosphere  (to  check  the  greenhouse  hypothesis). 

Veneras  4  and  6  also  obtained  unexpected 
results  in  plasma  and  magnetic  measurements. 
They  discovered  a  shock  wave  in  the  solar  wind 
near  Venus  like  the  one  near  Earth.  The  shock 
front  of  Venus,  however,  was  much  closer  to 
its  surface.  Before  the  spaceflight  to  Venus, 
scientists  hypothesized  that  the  number  density 
of  charged  particles  in  Venus'  ionosphere 
could  exceed  by  three  orders  of  magnitude  the 
number  density  of  charged  particles  in  the  main 
peak  in  the  terrestrial  ionosphere.  Ion  number 
densities  that  Venera  4  measured  during  its 
descent  on  Venus'  nightside  did  not  confirm 
that  suggestion,  nor  did  Mariner  5's  radio- 
occultation  observations  about  electron  number 
densities  on  the  ionosphere's  nightside  and 
dayside.  In  Venus'  ionosphere,  the  maximum 
number  density  of  charged  particles  was  about 
the  same  as  on  Earth.  Mariner  5  observed  a 
distinct  upper  boundary  of  the  dayside  iono- 
sphere at  an  altitude  of  500  km  (310  miles). 
Within  the  boundary,  the  electron  number 
density  decreased  by  two  orders  of  magnitude 
within  an  altitude  range  of  only  50  to  100  km 
(31  to  62  miles).  The  boundary  was  similar  to 
the  plasmapause — the  upper  bound  of  Earth's 
thermal  plasma  envelope.  Because  of  this 
similarity,  scientists  gave  the  name  ionopause 
to  the  Venus  phenomenon.  However,  Earth's 
plasmapause  is  much  farther  from  the  planet's 
surface,  roughly  20,000  km  (12,428  miles). 

Although  large-scale  features  typical  of  solar- 
wind  flow  around  both  Venus  and  Earth  are 
similar,  the  magnetic  field  Venera  4  first 
measured  near  the  planet  seemed  insignifi- 
cant— only  about  10  gamma  (10 -4  gauss)  at  an 
altitude  of  200  km  (124  miles).  The  surface 
magnetic  field  in  Earth's  equatorial  region  is 
about  50,000  gamma.  Until  recently,  it  had 
been  thought  that  Venus'  intrinsic  magnetic 
field  might  play  a  significant  role  in  forming 
the  pattern  of  solar-wind  flow  around  the 
planet,  as  it  does  in  the  case  of  Earth. 

Operating  an  automatic  interplanetary  probe 
in  Venus'  hot  and  dense  atmosphere  was 
technically  difficult.  Nevertheless,  in  the  1960s, 
a  team  of  scientists  designed  spacecraft  for 
Venus  research.  The  academician  S.  P.  Korolev 
and  then  G.  N.  Babakin,  Corresponding  Mem- 
ber, U.S.S.R.  Academy  of  Sciences,  headed  the 
team.  NASA  lauched  Pioneer  Venus  1 1  years 
after  Venera  4,  almost  at  the  same  time  as 
Veneras  11  and  12  were  launched. 

It  often  happens  in  science  that  the  solution  to 
one  problem  leads  to  new,  more  complicated 
problems.  Spaceflights  to  Venus  were  no 
exception.  They  showed  that  climatic  and 
atmospheric  conditions,  so  similar  to  Earth  for 
some  physical  parameters,  are  generally  quite 
different  from  those  on  Earth.  What  are  the 
reasons  for  these  differences?  Can  the  climate 
and  composition  of  Earth's  atmosphere 
experience  the  same  changes  in  the  foreseeable 
future?  If  so,  what  would  cause  such  changes: 
altered  external  conditions,  environmental 
pollution,  or  something  else?  Such  questions 
prompt  many  scientists  throughout  the 
world  to  consider  exploration  of  Venus  a 
top-priority  task. 

Venus  can  be  a  natural  "cosmic  laboratory" 
for  studies  in  comparative  planetology.  The 
value  of  such  research  becomes  more  apparent 
because  it  is  impossible  to  realize  experiments 
on  such  a  scale  under  Earth  conditions. 

Any  planet's  atmosphere  is  a  complex  system 
with  many  interactions  and  feedbacks.  Its 
composition,  for  instance,  is  determined  by 
how  and  under  what  conditions  the  planet 
formed,  and  by  outgassing  processes  from  its 
solid  body.  Other  factors  include  reactions 
among  atmospheric  gases,  the  upper  atmo- 
sphere's structure  (from  which  light  gases 
escape  into  the  interplanetary  space),  and  so  on. 
The  character  and  rate  of  many  atmospheric 


processes  depend  on  temperature,  which  in 
turn  depends  on  the  atmosphere's  composi- 
tion. The  latter  consideration  is  most  essential 
for  Venus.  The  gaseous  and  aerosol  composi- 
tion of  the  Venusian  atmosphere  allows  some 
solar  radiation  to  penetrate  down  to  the 
surface.  The  opacity  of  the  atmosphere  is  high, 
however,  for  infrared  radiation.  As  a  result,  the 
surface  temperature  remains  high.  The  phe- 
nomenon, which  we  call  the  greenhouse  effect, 
is  much  more  conspicuous  on  Venus  than  on 
Earth.  On  Earth,  the  greenhouse  effect  adds 
about  35°C  (63°F)  to  the  surface  temperature. 

A  fuller  understanding  of  what  is  taking  place 
on  Venus  required  sophisticated  chemical 
analyses  of  the  atmosphere  and  an  exact 
knowledge  of  the  altitudes  and  spectral  regions 
where  solar  radiation  is  absorbed.  Scientists 
also  needed  to  study  the  nature  of  the  clouds 
that  prevent  astronomers  from  seeing  the 
lower  layers  of  the  atmosphere. 

After  the  first-generation  Venera  probes  made 
plasma  and  magnetic  measurements,  scientists 
were  faced  with  many  new  problems.  With 
theories  and  concepts  existing  at  the  time,  it 
might  have  been  possible  to  find  solutions  to 
some  of  the  problems.  In  particular,  scientists 
wanted  to  explain  the  weak  intrinsic  magnetic 
field  near  Venus.  For  example,  they  could  use 
theories  of  how  magnetic  fields  originate  and 
maintain  themselves  near  planets  on  the  basis 
of  planetary  dynamos.  These  theories  predict 
that  a  planet,  if  it  has  an  intrinsic  magnetic  field, 
must  rotate  rapidly  and  have  a  liquid,  con- 
ducting core.  Scientists  had  used  close  values 
of  mean  densities  of  terrestrial  planets  to  build 
similar  models  of  their  inner  structures. 
Consequently,  planetologists  could  attribute 
Venus'  absence  of  an  intrinsic  magnetic  field 
to  its  slow  rotation  (about  243  terrestrial  days). 

Scientists  observed  shock  waves  near  both 
Venus  and  Earth.  But  Venus,  they  knew,  had  a 
much  weaker  intrinsic  magnetic  field  than 
Earth.  What  is  the  obstacle — different  from 
Earth's — that  retards  the  solar  wind  and  forms 
a  shock  wave  near  Venus? 

Indeed,  a  strong  intrinsic  magnetic  field  pro- 
tects Earth,  its  atmosphere,  and  ionosphere 
against  the  solar  wind's  direct  effect.  However, 
for  Venus,  the  solar  wind  could  interact 
directly  with  its  atmosphere  and  ionosphere, 
causing  ionization,  compression,  and  heating 
of  the  ionosphere  and  atmosphere.  The  solar 
wind,  flowing  around  the  planet's  conducting 
ionosphere,  together  with  the  interplanetary 
magnetic  field,  could  induce  electric  currents 
in  the  ionosphere  and  thus  produce  induced 
magnetic  fields.  If  these  induced  fields  are 
strong  enough,  they  could  brake  the  solar 
wind  and  form  an  induced  magnetosphere, 
rather  than  an  intrinsic  one,  near  the  planet. 

All  these  assumptions  rested  on  the  observed 
similarities  and  differences  in  the  solar  wind's 
pattern  flowing  around  Venus  and  Earth,  and 
they  had  to  be  verified.  Much  more  complex 
and  accurate  measurements  were  needed. 

To  conduct  more  detailed  experiments  in  the 
deep  layers  of  Venus'  atmosphere,  interplan- 
etary probes  needed  heavier  and  more  sophis- 
ticated instruments.  More  importantly,  the 
vast  amount  of  data  gathered  by  the  instru- 
ments had  to  be  transmitted  back  to  Earth. 
Accordingly,  the  first-generation  probes,  which 
had  not  been  intended  to  deal  with  such 
problems,  were  succeeded  by  Veneras  9 
through  12  (see  Figure  7-1).  Whereas  the  ear- 
lier probes  had  entered  the  Venusian  atmo- 
sphere in  their  entirety,  the  new  Venera  probes 
separated  into  an  orbiter  and  a  lander  some 
time  before  landing.  Depending  on  mission 
profile  and  ballistics,  the  orbiter  either  became 

an  artificial  satellite  of  Venus  (Veneras  9  and  10) 
or  it  flew  past  the  planet  and  entered  an  orbit 
around  the  Sun  (Veneras  11  and  12)  (Figure  7-2). 
The  orbiters  carried  instruments  to  study  the 
planet's  radiation  at  various  wavelengths,  the 
interplanetary  plasma  and  magnetic  fields,  and 
to  conduct  astronomical  observations. 

In  1975,  Veneras  9  and  10  splendidly  demon- 
strated the  capabilities  of  a  new  generation  of 
spacecraft.  For  the  first  time,  a  panoramic  view 
of  another  planet  was  transmitted  from  its  sur- 
face to  Earth  (Figure  7-3).  A  series  of  investiga- 
tions looked  at  the  atmosphere's  optical  prop- 
erties. They  determined  the  general  features  of 
the  cloud  structure.  The  clouds  are  in  a  layer 
about  20  km  (12  miles)  thick,  with  a  lower 
boundary  at  an  altitude  of  50  km  (31  miles). 
Radiation  fluxes  were  measured  in  several 
spectral  regions  and  the  water  vapor  content 
was  derived  from  the  intensity  of  the  absorp- 
tion band.  Scientific  equipment  onboard  the 
orbital  vehicles — Venus'  first  artificial  satel- 
lites— produced  important  results. 

A  series  of  plasma  and  magnetic  radio-occultation 
observations  (Veneras  9  and  10  orbiters)  made 
it  possible  to  study  in  detail  the  solar-wind 
flow  pattern  around  the  planet,  and  discover  a 
plasma-magnetic  tail  of  the  planet.  The  obser- 
vations also  allowed  scientists  to  investigate 
the  character  of  the  magnetic  field  and  the 
properties  of  the  dayside  and  nightside  iono- 
sphere, and  .to  identify  atmospheric  ionization 
sources  in  the  planet's  deep  optical  umbra. 

Analyses  of  Veneras  9  and  10  experimental  data 
indicated  new  problems.  But  expertise  in 
designing  sophisticated  scientific  equipment 
that  could  operate  under  very  difficult  condi- 
tions (enormous  decelerations,  high  tempera- 
tures and  pressures)  solved  most  of  them  in  the 
Veneras  11  and  12  probes  that  reached  Venus 
late  in  1978.  The  construction  of  a  huge,  70-m 

(230-ft)  diameter  parabolic  reflector  at  the 
Deep  Space  Communication  Center  also 
greatly  improved  data  reception  from 
the  landers. 

Recent  scientific  results  from  the  new  genera- 
tion of  Soviet  Venera  probes  are  discussed  in 
the  sections  that  follow.  Table  7-1  summarizes 
launch  dates,  descent  module  landing 
coordinates,  and  other  data. 

Chemical  Composition  of  the 
Venusian  Atmosphere 

Until  1967,  scientists  assumed,  because  of  the 
planet's  similarity  to  Earth,  that  the  main 
chemical  in  Venus'  atmosphere  was  nitrogen. 
Besides  nitrogen,  scientists  expected  to  find  a 
small  amount  (1%  to  10%)  of  carbon  dioxide, 
whose  absorption  bands  they  had  observed  as 
far  back  as  the  1930s.  But  even  simple  chemi- 
cal sensors  on  the  first  Venera  probes  proved 
the  very  opposite  to  be  the  case.  The  most 
abundant  gas  in  the  atmosphere  is  carbon 
dioxide  (96.5%  according  to  estimates), 
whereas  nitrogen  makes  up  just  over  3%.  At 
the  time,  it  was  impossible  to  get  reliable 
information  about  the  content  of  the  atmo- 
sphere's many  small  constituents:  water  vapor, 
oxygen,  carbon  monoxide,  sulfur  compounds, 
and  noble  gases.  These  constituents  play  a 
tremendous  part  in  the  life  of  the  atmosphere. 
They  absorb  solar  and  thermal  radiation  (the 
greenhouse  effect),  participate  in  chemical 
reactions,  condense  to  form  cloud  layer  par- 
ticles, and  also  contribute  to  other  processes. 

The  abundance  of  noble  gases  and  their  iso- 
topes is  of  particular  interest.  These  isotopes 
fall  into  two  groups:  radiogenic  isotopes  and 
primordial  isotopes.  The  radioactive  decay  of 
elements  formed  radiogenic  isotopes.  Primordial 
isotopes  have  survived  since  the  formation  of 
the  Solar  System's  planets  some  4.5  billion 
years  ago.  From  the  absolute  and  relative 


Venera  9 

Venera  3 


Figure  7-1.  In  the  two 
decades  that  the  United 
States  sent  four  spacecraft 
to  Venus,  the  Soviets 
attempted  29  missions  (15 
were  successful).  Although 
some  of  the  failures  were 
never  officially  admitted, 
U.S.  or  European  sources 
detected  them.  These  seven 
illustrations  show  the  evolu- 
tion of  the  Soviet  spacecraft 
to  explore  Venus.  It  came 
from  many  sources  and  was 
not  a  part  of  the  Soviet 
authors' contribution  to  this 
chapter.  We  have  included  it 
to  place  the  U.S.  and  Soviet 
missions  in  perspective. 

Figure  7-2.  Landing  scheme  of 
the  Soviet  second  generation 
automatic  spacecraft 
(Veneras9,  10,  11,  12). 

1)  Interplanetary  spacecraft 
on  Venusian  orbit. 

2)  Separation  of  descender 
and  arbiter  two  days  before 
the  landing. 

3)  Entry  into  the  Venusian 

4)  Deployment  of  auxiliary 
and  displacement  parachutes. 

5)  Jettisoning  of  hatch. 

6)  Deployment  of  decelera- 
ting parachute  at  66  to 

62  km  (4 1  to  38.5  miles)  and 
beginning  of  telemetry  data 

7)  jettisoning  of  lower  sector 
of  thermal  protection  shell  and 
jettisoning  of  decelerating 
parachute  at  about  48  km 
(30  miles)  altitude. 

8)  Landing  and  data  trans- 
mission to  Earth  via  the 
flyby  bus. 

Typical  Venera  Lander 


content  of  primordial  isotopes,  we  can  gain 
some  insight  into  the  Solar  System's  history,  in 
particular,  about  conditions  in  which  the 
protoplanetary  nebula  gave  rise  to  the  planets, 
and  about  their  formation  process.  Argon 
isotopes  will  be  discussed  as  an  example. 

For  fine  chemical  analysis  of  atmospheric  gases, 
Soviet  investigators  used  a  mass  spectrometer, 
a  gas  chromatograph,  and  an  optical  spectrom- 
eter. (The  mass  spectrometer  takes  microscopi- 
cally small  gas  samples,  ionizes  them,  and 
sorts  them  according  to  their  mass  with  a  high 
frequency  electric  field.)  A  group  of  scientists 
headed  by  Vadim  Istomin  (Institute  of  Space 
Research,  U.S.S.R.  Academy  of  Sciences) 
conducted  the  mass  spectrometer  experiment. 
The  instruments  (Figure  7-4)  on  both  vehicles 
switched  on  at  an  altitude  of  about  24  km 
(15  miles)  and  operated  until  touchdown. 
These  instruments  scanned  the  mass  range 
from  10  to  105  atomic  units  in  7  seconds.  The 
gas  sampling  time  was  under  5xlO-3  seconds, 
and  the  sampling  rate  was  once  every  3  min- 
utes. The  instruments  took  a  total  of  22  sam- 
ples and  transmitted  about  200  mass  spectra  to 
Earth.  The  mass  spectrum  in  Figure  7-5  is  an 
average  over  7  of  200  mass  spectra. 

The  mass  spectra  show  several  peaks.  These 
peaks  correspond  to  the  molecules  carbon 
dioxide  and  nitrogen,  and  the  atoms 
carbon-12,  carbon-13,  oxygen-16,  oxygen-18, 
and  nitrogen-14  (from  decomposition  of 
carbon  dioxide  and  nitrogen  molecules  inside 
the  instrument).  Also  corresponding  to  peaks 
are  three  noble  gases:  neon,  argon,  and 
krypton.  Quantitative  data  appear  in  Table  7-2. 
The  presence  of  krypton  (about  6.5xlO-s%)  is 
noteworthy.  Instruments  on  the  Pioneer 
Venus  probe  detected  no  krypton. 

In  Istomin's  experiment,  every  single  record  of 
the  mass  spectrum  shows  krypton.  Estimates 
averaged  over  tens  of  records  showed  that  the 
relative  abundances  of  the  main  krypton 
isotopes  with  atomic  weights  84,  86,  83,  and 
82  are  comparable  to  those  on  Earth.  The 
argon  results  were  extremely  surprising.  The 
radiogenic  isotope  argon-40  and  the  primor- 
dial argon-36  are  present  in  Venus'  atmo- 
sphere in  equal  amounts.  On  Earth,  argon-40 
is  300  times  more  abundant  than  argon-36. 

A  full  explanation  of  this  anomaly  is  a  matter 
for  the  future,  but  M.  Izakov  (Institute  of  Space 
Research)  has  proposed  an  elegant  hypothesis. 

Figure  7-3.  Panoramic  view 
of  the  Venusian  surface  at 
291  east  longitude,  32  north 
latitude,  relayed  by  Venera  9 
descent  module.  Numerous 
stone  blocks  with  sharp  edges 
are  around  the  spacecraft,  a 
fact  testifying  to  their  com- 
paratively young  age.  On  the 
planet's  surface  at  the  land- 
ing site,  much  small-grained 
substance  resembling  dust 
or  sand  is  visible.  After  the 
lander's  impact,  a  dust  cloud 
rose  that  registered  on  a 
photometer  for  a  few  minutes. 


Figure  7-4.  A  general  view  of  the 
mass  spectrometer  carried  by  the 
Venera  spacecraft. 


He  assumes  that  Venus  derived  the  greater  part 
of  its  atmosphere  from  the  protoplanetary 
nebula.  Earth  (and  Mars)  captured  relatively 
little  gaseous  material  from  it,  and  most  of  their 
atmospheres  were  outgassed  from  their  interiors. 
According  to  this  hypothesis,  the  meteorite  and 
asteroid  accumulation  process,  which  gave  rise 
to  all  the  planets  4.5  billion  years  ago,  pro- 
ceeded more  rapidly  for  Venus.  This  happened 
because  the  planet  is  closer  to  the  Sun,  and  the 
meteorite  bodies  were  denser  there.  The  cap- 
ture of  gas  also  was  more  rapid.  Before  the  new 
data,  scientists  believed  the  atmospheres  of  the 
Earth  group  of  planets  (Venus,  Earth,  and  Mars) 
were  of  secondary  origin,  formed  by  degassing 
from  their  interiors.  The  argon-36  anomaly  for 
Venus,  however,  casts  doubt  on  this. 

The  atmosphere  of  Venus  was  also  chemically 
analyzed  by  the  Sigma  gas  chromatograph 
(Figure  7-6).  Lev  Mukhin  of  the  Institute  of 
Space  Research  supervised  this  experiment. 
(Gas  chromatographic  analysis  is  based  on 
different  degrees  of  adsorption  of  various  gases 
by  porous  substances.  The  heart  of  the  gas 
chromatograph  is  a  column  filled  with  a 

specific  sorbent.  The  instrument  pumps  an 
atmospheric  gas  sample  through  the  column. 
There  the  mixture  separates  into  individual 
components.  Various  constituents  of  the 
mixture  leave  the  column  one  by  one,  and  a 
special  ionization  detector  records  them.) 

A  chromatograph  was  also  installed  onboard 
the  Pioneer  Venus  Large  Probe  (V.  Oyama  at 
Ames  Research  Center  supervised  this  experi- 
ment). Oyama  (1979)  reported  that  no  carbon 
monoxide  was  found,  but  Venus'  atmosphere 
contained  a  large  amount  of  molecular  oxygen 
(exceeding  the  upper  limit  from  the  Soviet 
experiment).  Oyama  later  reported  (1980)  that 
he  had  misidentified  the  relevant  chromato- 
graphic peaks,  and  the  missing  carbon  monox- 
ide was  found. 
Oyama's  data 
revealed  another 
aspect  that  has  not 
been  explained:  the 
presence  of  rela- 
tively large  amounts 
of  water  vapor — 
approximately  0.5% 
at  an  altitude  of 
44  km  (27  miles) 
and  0.1%  at  24  km 
(15  miles). 

Water  absorbs  light 
in  several  spectral 
bands,  some  of 
which  (7200,  8200, 
and  9500  angstroms) 

are  quite  distinct  in  the  spectra  from  the  optical 
spectrophotometer  (Figure  7-7)  onboard  the 
Veneras  11  and  12  descenders.  (V.  Moroz  super- 
vised this  experiment.)  From  the  bands'  inten- 
sity, scientists  could  determine  water  content 
in  the  Venusian  atmosphere  at  different  alti- 
tudes. This  quantity  proved  very  small  (2x10-3% 
near  the  surface  and  2x10-2%  at  50  km,  or 








I       I      I      I     I     I     I 

I       I      I      I      I 

I      I      I 

12       14       16     18    20 



52       60 



Figure  7-5.  Averaged  mass 
spectrum  (the  sum  of  seven 
separate  spectra)  obtained  in  the 
regime  of  noble  gas  analysis. 

31  miles).  Oyama's  experiments  had  yielded  a 
quantity  several  orders  of  magnitude  greater. 

Parallel  measurements  with  a  chromatograph 
and  a  mass  spectrometer  provided  indepen- 
dent control  of  the  results.  The  Venera  12 
chromatograph  did  not  detect  water  vapor. 
From  this  fact,  it  follows  that,  at  an  altitude 
below  24  km  (15  miles),  water  vapor  content  is 
below  0.01%.  The  Veneras  11  and  12  mass 
spectrometers  registered  a  slight  excess  in  the 
oxygen- 16  mass  peak  as  compared  with 
oxygen- 18  (if  the  oxygen- 18/oxygen- 16  ratio  is 
assumed  to  be  exactly  equal  to  Earth's).  Note 
that  oxygen- 18  and  oxygen- 16  are  formed  in 
the  instrument  from  carbon  dioxide.  If  this 
excess  is  due  to  the  water  contribution  (the 
molecular  weight  of  water  also  is  18),  the  water 
vapor  abundance  correlates  reasonably  well 
with  the  optical  measurements. 

There  is  a  simple  way  to  verify  whether  the 
quantity  of  water  vapor  varies  from  site  to  site. 
The  height  dependence  of  temperature  that 

Pioneer  Venus'  Large  Probe  obtained  can  be 
compared  with  infrared  radiation  fluxes 
measured  by  the  same  vehicle.  This  compari- 
son makes  it  possible  to  calculate  the  mean 
absorption  coefficient  for  thermal  planetary 
radiation  (into  which  the  diffuse  solar  light 
penetrating  deep  in  the  atmosphere  is 

Figure  7-6.  Cos  chromatograph 
carried  by  the  Veneras  1 1 
and  12  spacecraft. 


Table  7-2.  Chemical  Composition  of  the  Atmospheres  of  Venus  and  Earth 


Content  by  volume,  % 



Carbon  dioxide 






Water  vapor 






Carbon  monoxide 



Sulfur  dioxide 


Hydrogen  chloride 

4  X  1  0-5° 


Hydrogen  fluoride 

5  X  1  0-7C 






2X  10-4 




Noble  gases: 








1  .5  X  1  0-2 




1.1  X  TO"* 

Mean  molecular  weight 



aMixing  ratio  near  surface.  At  an  altitude  of  50  km,  it  is  an  order  of  magnitude  higher;  at  70  km, 

an  order  of  magnitude  less. 

Mixing  ratio  below  20  km;  at  70  km,  it  is  four  orders  of  magnitude  less. 
cMixing  ratio  above  60  km  (only  the  data  for  ground-based  spectroscopy  available). 
dGaseous  sulfur  is  meant  (molecules  $2,  $3,  $4,  $5,  Sg,  $7,  and  Sg);  estimate  refers  to  altitudes 

below  40  km. 

transformed).  This  coefficient  depends 
strongly  on  atmospheric  water  vapor  concen- 
tration. The  calculated  water  vapor  concentra- 
tion was  found  to  correspond  closely  with 
optical  measurements. 

The  total  amount  of  water  vapor  in  Venus' 
atmosphere  appears  to  be  disastrously  small.  If 
the  planet's  entire  water  vapor  (2x10-3%) 
condensed,  it  would  form  a  liquid  layer  no 
more  than  1  cm  thick.  Obviously,  there  can  be 
no  seas,  oceans,  and  liquid  water  on  Venus' 
surface — the  temperature  is  too  great  for  that. 
All  of  Venus'  water  is  either  in  its  crust  or  in  its 
atmosphere.  This  is  yet  another  anomaly,  no 
less  odd  than  the  argon-36/argon-40  ratio. 

There  is  nothing  extraordinary  about  the  atmo- 
sphere's high  carbon  dioxide  concentration. 

Almost  all  of  Earth's  carbon  dioxide  is  bound 
up  in  carbonates.  On  Venus,  all  carbon 
dioxide — because  of  the  high  temperature  and 
absence  of  liquid  water — is  in  the  atmosphere. 
Total  amounts  of  carbon  dioxide  on  both 
planets  are  roughly  equal.  But  the  concentra- 
tion of  water  on  Venus  presents  a  problem. 
Three  explanations  are  possible:  (1)  Venus 
formed  with  less  water;  (2)  at  the  early  stages 
of  evolution,  water  vapor  dissociated,  hydro- 
gen escaped  into  the  interplanetary  space,  and 
oxygen  vanished  through  chemical  reactions; 
and  (3)  water  is  bound  up  in  minerals  (where 
there  are  rocks  that  retain  water  very  well  at 
high  temperatures). 

Solar  Radiation  and  Clouds  in  Venus' 

Both  the  Veneras  11  and  12  landers  carried 
spectrophotometers.  From  an  altitude  of  65  km 
(40  miles)  until  touchdown  on  Venus,  they 
registered,  for  the  first  time,  the  daylight  sky 
spectrum  and  the  angular  distribution  of 
brightness  at  10-second  intervals.  These 
measurements  showed  that  a  large  amount  of 
solar  radiation  reaches  the  planet's  surface. 
Significantly,  this  is  scattered  rather  than 
direct  sunlight.  Since  the  cloud  cover  at  60  to 
70  km  (37  to  43  miles)  scatters  solar  radiation, 
an  observer  could  not  see  the  Sun  from  Venus' 
surface  nor  from  an  altitude  of  55  km 
(34  miles).  In  terms  of  energy,  it  is  unimpor- 
tant what  sort  of  radiation  penetrates  Venus' 
atmosphere — direct  or  scattered.  An  evaluation 
of  solar  energy  reaching  the  surface  (3%)  and 
Venus'  thermal  radiation  confirmed  a  pro- 
nounced greenhouse  effect.  This  effect  results 
in  high  temperatures  in  the  atmosphere's  deep 
layers  and  at  the  Venusian  surface.  The  observa- 
tion confirms  the  hypothesis  that  Carl  Sagan  put 
forward  as  far  back  as  1962. 

According  to  Veneras  11  and  12  data,  the 
energy  distribution  in  the  scattered  sunlight 
spectrum  changes  as  the  probe  penetrates 
deeper  into  the  atmosphere.  Just  as  on  Earth, 
the  effect  results  from  two  types  of  scattering. 
The  first  is  aerosol  scattering  of  light  by  cloud 
particles.  The  second  is  Rayleigh  scattering  by 
carbon  dioxide  and  nitrogen  molecules.  The 
probes  also  detected  light  absorption  in  ultra- 
violet, which  probably  belongs  to  gaseous 
sulfur  molecules. 

There  are  several  layers  of  clouds  in  Venus' 
atmosphere  at  altitudes  from  50  to  70  km 
(31  to  43  miles).  Their  boundaries  are  distinct 
in  the  curves  showing  the  decrease  in  scattered 
sunlight  intensity  with  the  probe's  descent 
(Figure  7-8). 







E    80 














Ground-based  observations  fixed  the  approxi- 
mate position  of  the  cloud  cover's  upper 
boundary.  Veneras  9  and  10  nephelometers 
and  photometers,  however,  first  observed  the 
lower  boundary. 

Veneras  9  and  10  nephelometer  experiments 
(M.  Marov,  Institute  of  Applied  Mathematics, 
U.S.S.R.  Academy  of  Sciences)  made  it  possible 
not  only  to  determine  the  cloud  cover's  lower 
boundary,  but  also  to  estimate  cloud  particle 
concentration,  size,  and  the  atmosphere's 
refractive  index.  To  a  limited  extent,  the 

Figure  7-7.  Scattered  solar 
radiation  spectrum  in  deep 
layers  of  Venus'  atmosphere. 
Venera  1 1  's  descent  module 
obtained  the  data.  Numbers 
along  the  curves  indicate 
altitudes  in  kilometers.  Note 
how  the  lines  for  water  (H2O) 
and  carbon  dioxide  (CO2) 
became  more  dense  as  the 
probe  descended.  These 
spectra  proved  to  be  a  very 
good  source  of  data  on  the 
water  vapor  content  in 
Venus'  atmosphere. 


Figure  7-8.  Radiation  intensity 
from  the  zenith  as  a  function  of 
altitude  for  some  wavelengths. 
Venera  1 1  's  descent  module 
obtained  the  data.  Symbols 
along  the  curves  indicate  wave- 
lengths. The  sharp  change  in 
the  steepness  of  the  curves  at  an 
altitude  slightly  less  than  50  km 
(30  miles)  is  a  result  of  crossing 
the  lower  cloud  layer  boundary. 










A  (jam) 

O  1.000  D  .500 

<C>     .720  A  .450 

V     .635 


Venera  1 1  mission  repeated  these  observa- 
tions. The  Pioneer  Venus  Large  Probe  enabled 
R.  Knollenberg  and  D.  Hunten  to  study  in 
great  detail  the  particle-size  distribution. 

Venusian  clouds  are  relatively  transparent.  The 
meteorological  visibility  inside  the  clouds  is 
several  kilometers.  There  are  three  layers.  The 
upper  layer  is  at  57  to  70  km  (35  to  43  miles), 
the  middle  at  52  to  57  km  (32  to  35  miles), 
and  lower  at  49  to  52  km  (30  to  32  miles). 
Particles  are  of  three  types:  large  (7  microns  in 
diameter),  medium-sized  (2  to  2.5  micron), 
and  small  (average  diameter  0.4  micron).  Only 
small  and  medium-sized  particles  are  present 
in  the  upper  layer.  The  other  two  layers  have 
all  three  particle  types.  Large  particles  account 
for  no  less  than  90%  (in  terms  of  mass)  of  the 
entire  cloud  cover. 

The  composition  of  Venusian  clouds  has  long 
baffled  scientists.  The  simpler  hypotheses, 
based  on  Earth  analogies  (liquid  or  frozen 
water,  mineral  dust),  were  discarded  when 

ground-based  observations  yielded  data  on  the 
optical  properties  of  the  cloud  particles.  Since 
there  is  hydrochloric  acid  in  Venus'  atmo- 
sphere, scientists  put  forward  yet  another 
hypothesis.  They  speculated  clouds  consisted 
of  hydrochloric  acid  droplets.  But  a  number  of 
considerations  made  it  necessary  to  abandon 
this  assumption,  too.  In  terms  of  optical 
properties,  a  suitable  candidate  is  sulfuric  acid 
(H2SO4)  which  is  present  as  tiny  droplets  in 
Earth's  stratospheric  clouds.  Sulfur  compounds 
reach  the  atmosphere  all  the  time  from  Earths' 
interior,  and  chemical  reactions  produce  par- 
ticles that  are  in  Earth's  stratospheric  clouds. 
An  analogy  appears  quite  reasonable  here, 
since  a  sulfur  compound  (SO2)  and  pure  sulfur 
in  the  gaseous  state  occur  on  Venus. 

Also  in  terms  of  refractive  index  and  the 
infrared  absorption  coefficient,  sulfuric  acid  is 
a  suitable  candidate  for  the  main  component 
of  Venusian  cloud  particles.  This,  however, 
does  not  account  for  the  planet's  yellowish 
color.  Scientists  have  suggested  that  the  clouds 

contain  larger  particles  of  solid  sulfur,  in 
addition  to  particles  of  concentrated  sulfuric 
acid.  Nephelometric  experiments  revealed  that 
only  small  and  medium-sized  particles  could 
consist  of  sulfuric  acid.  The  large  particles 
must  have  a  different  composition.  It  was 
originally  assumed  they  did  consist  of  sulfur. 

The  Venera  12  mission  included,  for  the  first 
time,  an  experiment  on  the  direct  chemical 
analysis  of  cloud  particles.  (Y.  Surkov,  Institute 
of  Analytical  Chemistry  and  Geochemistry, 
U.S.S.R.  Academy  of  Sciences,  conducted  the 
experiment.)  Cloud  layer  particles  were 
collected  on  special  filters  and  analyzed  with 
an  x-ray  fluorescent  spectrometer.  The  instru- 
ment subjected  a  sample  to  hard  radiation 
from  a  radioactive  source.  As  a  result,  the  inner 
electron  shells  of  atoms  (K-shells)  were  excited, 
which  generated  characteristic  x-rays  whose 
spectrum  was  recorded  and  used  to  identify 
the  sample's  composition.  In  fact,  the  compo- 
sition was  determined  only  at  the  element 
level  since  molecules  or  any  types  of  bonds 
could  not  be  determined.  At  altitudes  from 
about  61  km  (38  miles)  down  to  49  km 
(30  miles),  the  most  abundant  element  among 
cloud-cover  particles  is  chlorine.  Either  sulfur 
is  not  present  at  all  or  there  is  only  about  1/20 
as  much  sulfur  as  chlorine.  Thus,  it  appears 
that  the  cloud  cover's  large  particles  consist  of 
chlorine  compounds,  although  it  is  not 
apparent  which  specific  compounds  these  are. 

Winds,  Storms,  and  Night-Sky  Glow 

Ground-based  observations  had  already 
established  that  Venusian  winds  are  unusual. 
Near  the  upper  boundary  of  clouds,  the  speed 
of  fairly  regular  atmospheric  streams  is  nearly 
100  m/sec  (328  ft/sec).  These  swiftly  flowing 
atmospheric  masses  form  a  single  stream  as 
they  sweep  above  the  slower  atmospheric 
layers  and  solid  body  of  the  planet.  The  rota- 
tion period  of  the  planet's  body  is  very  long — 

243  Earth  days.  Venus'  rotation  is  retrograde, 
opposite  to  the  rotation  of  Earth  and  the  other 
planets  in  the  Solar  System.  The  clouds  move, 
together  with  the  upper  part  of  the  atmo- 
sphere, in  the  same  retrograde  direction, 
completing  one  rotation  in  4  days  at  an 
altitude  of  65  to  70  km  (40  to  43  miles). 

Measurements  of  the  lander's  descent  velocity 
made  it  possible  to  determine  the  wind  profile 
down  to  the  surface.  As  the  lander  approached 
the  planet's  surface,  the  wind  gradually 
subsided.  Within  the  last  10-km  (6-miles)  thick 
layer  of  atmosphere,  the  wind  speed  was  only 
about  1  m/sec  (3  ft/sec).  To  measure  wind 
velocity  on  the  surface,  the  Veneras  9  and  10 
landers  carried  conventional  wind  vanes. 

The  existence  of  clouds  in  the  atmosphere  and 
the  highly  intensive  dynamic  processes  that 
occur  there  made  it  quite  probable  that  storm 
phenomena  might  be  present.  The  objective  of 
experiments  that  L.  Ksanfomaliti  (Institute  of 
Space  Research)  supervised  was  to  find  effects 
in  Venus'  atmosphere  similar  to  terrestrial 
thunderstorms.  Storm  discharges  generate  low- 
frequency  electromagnetic  pulses.  Ksanfomaliti 
used  a  low-frequency  (8  to  100  kHz)  spectrum 
analyzer  with  an  external  antenna  in  the  experi- 
ment and  did,  in  fact,  observe  pulse  radiation 
similar  to  that  typical  in  Earth's  thunderstorms 
(Figure  7-9).  After  receiving  Veneras  11  and  12 
mission  results,  scientists  analyzed  the  night- 
side  observation  data  that  Veneras  9  and  10 
had  earlier  obtained.  It  turned  out  that  Venera  9 
had,  indeed,  registered  a  short-lived  glow  on 
Venus'  nightside.  The  glow  was  possibly 
storm-generated.  Estimates  suggest  that  the 
number  of  storms  on  Venus  could  be  even 
greater  than  on  Earth. 

For  a  long  time,  many  ground-based  observers 
have  noted  a  weak  nightglow  (the  ashen  light 
of  Venus).  It  seems  possible  that  this  effect 


arises  during  periods  of  particularly  high  storm 
activity.  Besides,  another  effect — a  constant 
night  airglow  undetectable  from  Earth — results 
from  chemical  reactions  in  the  upper  atmo- 
sphere. In  the  visible  spectrum,  this  airglow 
only  occurs  when  molecular  oxygen  bands 
are  excited  in  a  carbon  dioxide  rich  atmo- 
sphere such  as  Venus'.  Veneras  9  and  10 
orbiters  were  the  first  to  register  the  bands. 
(V.  Krasnopolsky,  Institute  of  Space  Research, 
supervised  the  experiment.) 

The  Sun's  ultraviolet  radiation  (in  the  hydro- 
gen and  helium  lines)  is  scattered  by  corre- 
sponding atoms  in  the  planets'  upper  atmo- 
sphere. The  excited  atoms  re-emit  ultraviolet 
quanta  and  produce  line-scattered  radiation. 
Measurements  of  its  intensity  can  be  converted 
to  hydrogen  and  helium  concentrations.  These 
lightest  of  elements  make  up  the  outermost 
portions  of  the  atmospheres  of  Earth,  Mars, 
and  Venus.  Veneras  11  and  12  flyby  probes 
each  carried  an  instrument  to  measure  radia- 
tion intensity  in  the  upper  atmosphere  in  10 
different  ultraviolet  intervals  of  the  spectrum, 
which  included  hydrogen  and  helium  lines 
and  lines  of  several  other  elements.  V.  Kurt 
(Institute  of  Space  Research)  supervised  the 
experiment,  which  also  involved  French 
physicists].  Blamont  and  J.  L.  Bertaux.  An 
analysis  of  the  high-quality  spectra  provided 
some  estimates  of  the  composition  and 
structure  of  Venus'  upper  atmosphere. 

Experiments  conducted  during  the  descent  of 
Veneras  1 1  and  12  into  Venus'  atmosphere 
studied  three  basic  problems:  fine  chemical 
analysis  of  atmospheric  gases,  nature  of  clouds, 
and  thermal  balance  of  the  atmosphere. 
Of  these,  the  chemical  composition  studies 
were  considered  the  most  essential.  All  the 
experiments  were  successful.  The  scientific 
instruments  on  the  Pioneer  Venus  probe  were 
similar  to  those  on  the  Venera  probes — a  gas 

chromatograph,  a  mass  spectrometer,  and 
some  optical  instruments.  A  comparison  of  the 
results  is  of  great  interest. 

In  April  1979,  Soviet  and  American  scientists 
who  had  participated  in  both  missions  met  at 
the  Institute  of  Space  Research,  U.S.S.R. 
Academy  of  Sciences,  Moscow.  During  that 
meeting,  they  compared  data  from  the  differ- 
ent probes  and  discussed  the  implications.  The 
meeting's  published  results  made  it  clear  that 
the  space  science  community  had  succeeded  in 
studying  the  fine  chemical  composition  of 
Venus'  atmosphere.  The  investigations  of  both 
the  Soviet  and  American  probes  had  cleared  a 
way  for  solving  the  mysteries  about  Venus. 

Solar-Wind  Interaction  with  Venus- 
Bow  Shock  and  Intrinsic  Field 

The  first  experimental  observations  of  Venus' 
bow  shock  were  obtained  from  descending  and 
flyby  trajectories  of  Venera  4,  Venera  6, 
Mariner  5,  and  Mariner  10.  The  properties  of 
the  plasma  were  measured  by  Venera  4  with 
charged-particle  traps.  K.  Gringauz,  Institute  of 
Space  Research,  U.S.S.R.  Academy  of  Sciences, 
headed  the  experiments.  S.  Dolginov  and  his 
colleagues  (Institute  of  Earth  Magnetism  and 
Radiowave  Propagation,  U.S.S.R.  Academy  of 
Sciences)  measured  the  magnetic  field. 

The  various  types  of  charged-particle  traps,  or 
wide-angle  detectors,  are  actually  a  system  of 
electrodes — a  collector  and  several  grids.  Vari- 
ous voltages — direct  current,  gradually  chang- 
ing direct  current,  and  alternating  current — are 
usually  applied  to  these  grids,  which  makes  it 
possible  to  analyze  the  trapped  particles  by 
their  energies  and  charge  signs.  Scientists 
observed  the  shock  wave  as  a  sharp,  simulta- 
neous increase  in  the  interplanetary  plasma 
and  amplitude  of  magnetic  field  fluctuations 
that  occurred  some  distance  from  Venus. 









36  kHz 


5  hr  44  min    45 


47  48 

Time  (^ 





33  32  31  30  29  28  27  26 

Probe's  altitude  above  a  6052-km  level 



Figure  7-9.  The  "GROZA" 
experiment  of  the  Venera  1 1 
descent  module  recorded  these 
radio  noise  bursts.  The  bursts 
are  plotted  against  altitude 
for  various  frequencies.  Light- 
ning strikes  in  the  planet's 
atmosphere  evidently  caused 
the  noise. 


Systematic  observations  of  the  interactions  of 
the  solar  wind  with  Venus  were  performed  with 
plasma  and  magnetic  instruments  onboard  the 
first  Venus  orbiters,  Veneras  9  and  10.  The 
plasma  properties  were  measured  with  wide- 
angle  analyzers,  Faraday  cups,  and  retarding 
potential  analyzers  (RPA)  (K.  Gringuaz,  Space 
Research  Institute)  and  with  narrow-angle 
detectors  and  electrostatic  analyzers  (O.  Vaisberg, 
Space  Research  Institute).  The  magnetic 
measurements  were  made  by  S.  Dolginov, 
Institute  of  Earth  Magnetism  and  Radiowave 

An  electrostatic  analyzer  is,  in  its  simplest 
form,  two  curved  concentric  plates  separated  by 
a  small  gap.  A  potential  difference  is  applied  to 
the  plates.  Particles  entering  the  gap  pass 
through  it  only  if  they  have  a  certain  energy/ 
charge  unit  ratio.  This  energy  corresponds  to 
the  applied  potential  difference.  By  applying 
different  potentials  to  the  plates,  an  energy 
spectrum  of  particles  can  be  obtained. 

Figure  7-10  shows  32  bow  shock  crossings  by 
Veneras  9  and  10.  These  data  are  from  the 
wide-angle  analyzers  and  show  the  mean  front 
position,  based  on  data  of  86  crossings  by 
Pioneer  Venus  (Slavin  et  al.).  The  shock  front 
position  near  Venus  is  close  to  the  surface — 
about  0.3  Venus  radius  in  the  frontal  subsolar 
area.  Two  circumstances  explain  the  differ- 
ences in  the  mean  front  positions  of  Soviet 
and  American  vehicles.  These  spacecraft 
crossed  the  front  at  different  latitudes,  and 
the  measurements  occurred  during  different 
phases  of  the  solar  activity  cycle. 

Veneras  9  and  10  also  took  measurements  with 
electrostatic  analyzers,  which  showed  that  the 
asymmetry  of  Venus'  bow  shock  was  linked  to 
the  solar  wind's  anisotropic  nature.  The  bow 
shock's  radial  distance  in  the  polar  direction  is 

approximately  2000  to  3000  km  (1243  to 
1864  miles)  greater  than  in  the  equatorial 

After  the  experiments  on  Venera  4  by 
S.  Dolginov  and  his  colleagues,  Venus'  mag- 
netic moment  was  initially  estimated  as  5  to 
8x10-21  gauss  cm3  (10  gamma  on  the  surface). 
After  reviewing  Veneras  9  and  10  data,  this 
estimate  was  lowered  and  the  intrinsic  field  on 
the  planet's  surface  was  assumed  not  to  exceed 
5  gamma. 

Magnetic  field  measurements  at  altitudes  from 
140  to  200  km  (87  to  124  miles)  showed  that 
most  field  values  did  not  exceed  the  threshold 
sensitivity  of  the  instrument,  or  2  gamma. 
Thus,  it  was  confirmed  that  Venus'  intrinsic 
magnetic  field  is  all  but  absent. 

Plasma  Magnetic  Tail 

All  trajectories  of  Soviet  vehicles  that  have 
landed  on  planets  or  put  artificial  satellites 
into  orbit  have  approached  planets  from  their 
nightside  and  have  allowed  observations  of 
the  planets'  wake  at  altitudes  greater  than 
1500  km  (932  miles).  Veneras  9  and  10  entered 
the  dayside  only  to  latitudes  above  32°.  These 
vehicles  penetrated  deep  into  the  planet's 
optical  umbra  and  allowed  detailed  measure- 
ments of  the  distribution  of  the  plasma  and 
magnetic  field.  Their  measurements  showed 
that  a  plasma-magnetic  tail  with  typical 
features  exists  near  Venus,  some  of  the  features 
being  similar  to  the  tail  of  Earth's  magneto- 
sphere.  In  particular,  the  oppositely  directed 
bundles  of  magnetic  field  lines  along  the 
Sun-planet  direction  were  present  on  Venus.  In 
other  words,  the  magnetic  field  component 
along  the  Sun-planet  direction  was  essentially 
higher  than  the  others. 

These  field  line  bundles  in  the  tails  were 
separated  in  the  layer  where  the  magnetic 
energy  density  had  a  deep  minimum.  This 
layer  is  similar  to  the  "neutral-sheet"  of  Earth's 
magnetosphere.  The  data  from  wide-angle 
analyzers  showed  that  plasma  properties  and 
distribution  in  the  tail  also  resemble  Earth's 
magnetotail.  At  the  tail  boundary  and  in  the 
transition  region,  a  characteristic  change  in 
differential  ion  spectra  was  observed  similar  to 
that  in  Earth's  boundary  layer,  or  plasma 

mantle.  The  plasma  features  deep  in  the  tail 
resemble  those  in  Earth's  plasma  sheath. 

Figure  7-11  shows  regions  of  solar-wind  inter- 
action with  Venus.  These  regions  include  the 
shock  wave,  the  transition  region  (A)  behind 
the  shock  front,  and  the  plasma-magnetic  tail. 
The  B-region  corresponds  to  the  corpuscular 
penumbra,  or  boundary  layer.  Data  from  elec- 
trostatic analyzers  also  indicated  a  tail  bound- 
ary that  separated  plasmas  with  different 

Verigin  et  al.  1978 
(5°  aberrated) 





X'  (Rv) 

Figure  7- 1 0.  Position  of  the 
shock  front  near  Venus 
(measured  by  Veneras  9 
and  1 0).  The  lengths  of  the 
short  curves  and  the  points 
show  the  parts  of  the  orbit 
where  the  spacecraft  crossed 
the  front.  The  solid  curve  shows 
the  average  position  of  the  front 
(determined  from  Pioneer  Venus 
data).  The  cylindrical  system 
of  coordinates  is  used  where 
the  X'-axis  is  oriented  to  the 
solar-wind  direction. 


properties.  Outside  this  boundary,  plasma  was 
evidently  of  solar-wind  origin  but  disturbed  by 
its  interaction  with  the  obstacle.  Inside  the 
boundary,  plasma  was  cooler  and  had  a 
smaller  bulk  velocity,  probably  an  accelerated 
or  heated  plasma  of  planetary  origin.  Such  a 
boundary  layer  could  appear,  and  its  properties 
would  resemble  the  boundary  of  two  liquids. 
One  boundary  moves  and,  because  of  viscous 
interaction  with  the  lower  liquid,  accelerates 
and  heats  it.  When  the  solar-wind  plasma  with 
the  frozen-in  magnetic  field  moves  relative  to 
the  ionospheric  plasma,  the  boundary  separat- 
ing these  liquids  can  be  unstable.  For  instance, 
the  boundary  begins  to  move  or  fluctuate 
because  of  increasing  solar-wind  pressure.  The 
bubbles  of  the  solar-wind  plasma  flow  are  then 
pressed  into  the  ionosphere,  tearing  away  from 
the  flow.  This  condition  also  could  occur  with 
ionospheric  plasma  rising  up  in  the  transition 
region.  A  variety  of  processes  cause  plasma 
instabilities,  smear  the  boundary,  and  dissipate 
solar-wind  energy  and  its  subsequent  transfer 
into  the  ionosphere. 

In  Figure  7-11,  the  region  extending  to  5  Venus 
radii  (C-region),  where  the  regular  ion  fluxes 
are  absent,  is  positioned  under  the  corpuscular 
penumbra,  which  is  the  corpuscular  umbra 
region  that  does  not  coincide  with  the  optical 
shadow  of  Venus.  The  behavior  of  the  electron 
fluxes  was  quite  different  from  the  measured 
ion  fluxes.  The  fluxes  were  everywhere,  including 
the  corpuscular  umbra.  Only  their  intensity 
decreased  (Figure  7-12),  and  the  character  of 
the  spectrum  changed;  that  is,  high-energy 
tails  appeared  in  the  spectrum.  Apparently, 
electrons  and  ions  inside  the  tail  were  sub- 
jected to  some  acceleration  processes. 

It  was  likely  that  in  the  far  tail  regions  of  Venus, 
the  boundary  layer  gradually  thickened  and 
merged  with  the  plasma  sheath  as  it  does  for 
Earth.  As  in  the  plasma  sheath  of  Earth's 

magnetosphere,  accelerated  ion  fluxes  with 
energy  greater  than  2  keV  (C-region  in 
Figure  7-11)  were  observed  near  the  neutral- 
sheet  plane.  These  fluxes  occurred  when  the  Bx 
component  of  the  magnetic  field  reversed  its 
sign  (x-axis  was  along  the  Venus-Sun  line — 
Figure  7-13).  Thus,  the  large-scale  pattern, 
magnetic  field  topology,  and  plasma  distribu- 
tion in  the  Venusian  tail  showed  a  striking 
resemblance  to  Earth's  magnetosphere. 

Nature  of  the  Obstacle  Forming  a 
Shock  Wave 

An  extended  tail  near  Venus  with  properties 
similar  to  those  in  Earth's  magnetosphere 
seems  rather  striking.  Before  the  Pioneer  Venus 
experiments,  this  tail  led  the  American  specia- 
list C.  T.  Russell  to  revise  the  magnetic  field 
estimates  that  Soviet  specialists  previously 
made.  He  increased  the  estimated  value  of 
Venus'  intrinsic  magnetic  field. 

More  careful  study  and  detailed  revisions  of 
the  data  for  magnetic  and  plasma  measure- 
ments near  Venus  have  begun.  An  analysis  of 
magnetic  measurement  data  on  Veneras  9 
and  10  showed  that  the  tail's  magnetic  field 
properties  had  one  essential  difference.  This 
difference  became  apparent  after  comparing 
data  the  two  spacecraft  obtained  simultaneously. 
One  spacecraft  was  in  undisturbed  solar  wind 
and  the  other  in  the  planet's  tail  region. 

During  each  measurement,  the  magnetic  field 
topology — two  field  line  bundles  stretched 
along  the  tail — was  preserved.  However,  in 
several  instances,  the  plane  of  the  neutral 
sheet  separating  these  bundles  changed  its 
orientation.  Sometimes  this  plane  was  located 
vertically,  almost  parallel  to  the  meridian 
plane,  but  this  is  not  typical,  for  example,  of 
Earth's  magnetotail.  By  comparing  the  mag- 
netic data  two  spacecraft  obtained  at  the  same 
time,  E.  Eroshenko  (Institute  of  Earth 

Magnetism  and  Radiowave  Propagation) 
showed  that  the  neutral-sheet  plane  in  the  tail 
always  remained  perpendicular  to  the  trans- 
verse component  of  the  interplanetary  mag- 
netic field.  It  rotated  with  the  rotation  of  this 
transverse  component. 

The  conclusion  is  that  the  measured  magnetic 
field  is  not  the  planet's  intrinsic  field.  Rather, 
it  is  the  field  of  the  "magnetic  barrier"  that 
currents  flowing  in  Venus'  conductive  iono- 
sphere induce.  In  other  words,  magnetic  field 
tubes  of  the  solar  plasma  flowing  around  the 
planet  encounter  an  almost  ideal  conductor: 

carries  the  ends  of  the  field  tubes  retarded  at 
the  frontal  part  of  the  planet.  The  tubes  drape 
the  planet  and  stretch  tail-like  on  the  night- 
side.  Thus,  the  field  line  bundles  elongate  in 
opposite  directions  on  the  two  sides  of  the 
planet.  The  orientation  of  the  plane  separating 
these  bundles  depends  on  the  orientation  of 
the  magnetic  field  in  the  undisturbed  solar 
wind.  In  the  simplest  case,  if  the  interplane- 
tary magnetic  field  vector  lies  in  the  ecliptic- 
horizontal  plane,  field  lines  of  the  tubes 
draping  the  planet  are  in  opposite  directions 
on  the  dawn  and  dusk  sides.  In  this  case  the 
neutral-sheet  plane  is  parallel  to  the  meridian 

Figure  7-11.  Schematic 
representation  of  the  near- 
planet  shock  wave  (dotted  line) 
and  Venus'  magnetosphere  from 
Veneras  9  and  1 0  data.  Arrows 
show  the  direction  of  the  solar- 
wind  plasma  flow.  The  A-region 
is  the  transition  layer  behind  the 
shock  front.  The  B-region  is  the 
boundary  layer.  The  C-region 
is  the  corpuscular  shadow.  The 
D-region  (solid  line)  is  the  mag- 
netosphere boundary.  The 
E-region  is  the  plasma  sheath 
which  contains  a  neutral  sheet 
separating  magnetic  field  lines 
directed  toward  each  other. 


the  ionosphere.  They  cannot  penetrate  it  and 
they  deform,  retarding  especially  strongly  near 
the  stagnation  subsolar  point  of  the  iono- 
sphere. The  magnetic  field  accumulates  at  the 
subsolar  region  and  forms  a  magnetic  barrier. 
Still  flowing  around  the  planet,  the  solar  wind 

plane.  If,  however,  the  interplanetary-field 
vector  is  in  the  meridian  plane  or  near  it,  the 
neutral-sheet  plane  will  either  partially  or 
completely  coincide  with  the  ecliptic  plane.  It 
is  very  difficult  to  distinguish  this  case  from 











i  i 
--   10 

1      \ 

1 i 
--  10 


i    i     r    i   r 

.51      234 
Ej  (keV) 

--   10 





i . 




3  4 

J I 

--   10 






0          10 






Figure  7-12.  Ion  energy 
spectra  that  Venera  1 0 
obtained  on  April  7  9, 1 976. 
The  spacecraft  measured 
the  intense  flows  of  ener- 
getic ions  (shaded  part  of 
the  0.42  spectrum)  in  the 
region  of  the  planet  tail 
where  the  magnetic  field 
Bx-component  changed  its 
sign  (Bx-component  turn  is 
shown  underneath  the  spec- 
tra between  0.42  and  0.44). 
These  flows  are  part  of  the 
plasma  sheath  of  the 
Venusian  tail. 

the  intrinsic  magnetosphere  tail,  with  the 
dipole  axis  near  the  polar  axis,  as  for  Earth. 

The  problem  remained  unsolved  for  currents 
that  form  the  induced  magnetosphere  flow. 
Another  unsolved  problem  was  how  an 
extended  induced  magnetic  tail  can  form. 

After  Veneras  9  and  10  experiments  and  on 
the  basis  of  research  by  American  investigators 
(P.  Cloutier  and  R.  Danniel),  E.  Eroshenko 
assumed  that  currents  are  induced  in  the  iono- 
sphere itself  and  are  mainly  in  its  maximum. 
The  region  from  the  ionosphere  maximum  to 
its  upper  boundary  is  200  to  300  km  (124  to 
186  miles)  on  the  dayside. 

Soviet  laboratory  simulation  experiments  (at 
the  Space  Research  Institute,  headed  by 
I.  Podgorny)  were  very  important  in  under- 
standing tail  formation  in  the  "induced" 
magnetosphere.  In  these  experiments  a 
Venusian  artificial  ionosphere  was  formed 

from  vaporization  products  of  a  wax  sphere 
placed  in  a  hydrogen  plasma  flow  with  a 
frozen-in  magnetic  field.  On  the  artificial 
ionosphere's  dayside,  a  sharp  boundary 
formed,  over  which  the  magnetic  field 
increased  with  the  "magnetic  barrier."  Field 
lines  were  parallel  to  the  ionospheric  bound- 
ary. Measurements  on  the  wax  sphere's 
nightside  showed  that  a  long  tail  forms  (up  to 
10  radii  of  the  sphere)  with  the  field  orien- 
tation in  the  tail  being  typical  of  the  observed 
Venusian  magnetosphere  (Figure  7-14). 


The  experiments  on  Pioneer  Venus  finally  con- 
firmed that  Venus  has  practically  no  intrinsic 
magnetic  field  and  that  a  magnetic  barrier 
forms  on  its  dayside. 

If  the  assumption  that  the  induced  current 
flow  inside  the  ionosphere  is  correct,  the  upper 
ionosphere  boundary  should  coincide  with  the 
magnetic  barrier's  upper  boundary.  However,  it 
does  not.  From  Pioneer  Venus  data,  the 

31 0<Vj<  360  km/sec 
ne/nCoo  =  Constant 








X  (103  km) 

Figure  7-13.  Distribution  lines 
for  constant  number  densities 
of  the  plasma's  electron  com- 
ponent near  the  region  where 
the  solar  wind  interacts  with 
Venus  (from  Veneras  9  and  1 0 
data).  Electron  measurements 
corresponding  to  velocities  of 
solar  wind,  v, ,  in  the  narrow 
interval  310  to  360  km/sec 
(193  to  224  miles/sec)  were 
chosen  for  the  analysis.  Num- 
bers along  the  lines  designate 
the  values  of  electron  number 
density,  he ,  relative  to  their 
values  in  the  solar  wind. 

barrier's  magnetic  field  usually  decreases 
sharply  on  the  upper  ionosphere  boundary,  or 
ionopause,  simultaneously  with  the  growth  of 
the  thermal  ionospheric  plasma's  concen- 
tration and  temperature.  That  is,  the  field 
behaves  as  if  there  is  a  conductor  carrying  a 
current  in  the  ionopause  region  at  50  to  100  km 
(31  to  62  miles).  Sometimes  Pioneer  Venus 
detected  high  values  of  the  magnetic  field 
inside  the  ionosphere  in  the  region  of  the 
main  maximum. 

It  is  evident  that,  in  the  ionosphere  itself, 
strong  currents  could  flow.  C.  T.  Russell  asso- 
ciated that  phenomenon  with  the  discovery  of 
magnetic  "flux  ropes"  in  Venus'  dayside  iono- 
sphere. American  specialists  (F.  Johnson  and 
W.  Hansen)  and  Soviet  specialists  (T.  Breus, 
E.  Dubinin  et  al.,  Space  Research  Institute) 
gave  qualitative  explanations  and  estimated 
flux-rope  characteristics. 

In  the  dayside  ionosphere,  a  special  set  of 
magnetic  field  tubes  from  the  magnetic 
barrier,  which  results  from  the  instability  of 
the  ionopause  as  it  fluctuates  due  to  varying 
solar-wind  pressure,  apparently  can  press  in 
the  ionosphere,  tear  off  the  solar-wind  flow, 
and  submerge  into  the  ionosphere.  With  these 

tubes  moving  in  such  a  manner,  the  field- 
aligned  current  can  twist  them  into  spirals  and 
make  their  cross  sections  more  compressed  as 
they  submerge  deeper  into  the  ionosphere. 
Pioneer  Venus  data  showed  that  the  entire 
dayside  ionosphere  was  often  filled  with  these 
flux  ropes  or  their  pieces. 

Dayside  and  Nightside  Ionospheres 
of  Venus 

Scientists  investigated  properties  of  Venus'  day- 
side  and  nightside  ionospheres  by  observing 
radio  occultations.  This  was  during  the  flybys  of 
Mariners  5  and  10,  Veneras  9  and  10,  and  the 
long  mission  of  Pioneer  Venus  Orbiter. 

In  1967,  ion  traps  on  Venera  4  made  the  first 
direct  measurements  of  the  ion  number  den- 
sity's upper  limit  in  Venus'  nightside  iono- 
sphere. In  1978-1979,  Pioneer  Venus,  using 
various  mass  spectrometers  and  plasma 
analyzers,  measured  ion  and  electron  number 
densities,  temperatures,  and  ionosphere  com- 
position. The  spacecraft  made  these  direct 
measurements  down  to  140  km  (87  miles)  on 
both  the  dayside  and  nightside  of  Venus. 


Figure  7-14.  Comparison  of 
laboratory  model  of  induced 
magnetosphere  (top  of  figure) 
with  the  field  topology  in  the 
tail  of  Venus'  magnetosphere 
measured  during  the  Veneras  9 
and  1 0  experiments.  Projection 
of  magnetic  field  vectors  appears 
in  the  system  of  coordinates 
rotating  together  with  the  inter- 
planetary magnetic  field  vector. 




Venus'  Dayside  Ionosphere 
Early  experiments  and  radio-occultation  obser- 
vations during  Mariner  5  and  10  flybys  of 
Venus  indicated  that  a  sharp  upper  boundary 
— an  ionopause — exists  on  electron  number 
density  profiles  in  the  dayside  ionosphere. 

The  ionopause  heights  of  these  profiles  were 
very  different:  500  km  (310  miles)  on  Mariner  5 
and  350  km  (217  miles)  on  Mariner  10.  The 
dynamic  pressure  of  the  undisturbed  solar 
wind  during  Mariner  10's  flyby  was  higher 
than  during  Mariner  5's.  Based  on  this  differ- 
ence, American  investigators  suggested  that 
the  solar  wind  could  compress  the  Venusian 
ionosphere  (S.  T.  Bauer).  As  a  result,  the  elec- 
tron number  density  profile  should  be  dis- 
torted, and  the  significant  flow  of  the  solar 
wind  could  then  penetrate  to  the  ionosphere. 
According  to  some  estimates  (C.  T.  Russell), 
the  value  of  the  incoming  solar-wind  flow 
could  be  30%  of  the  total  solar  flux.  As  a 
result,  the  shock  wave  might  "settle  down"  on 
Venus'  surface  and  become  attached  rather 
than  detached  (C.  T.  Russell).  As  the  data  from 
Veneras  9  and  10  showed  (N.  Savich,  Radio- 
electronics  Institute),  the  ionopause  has  a 
distinct  dependence  on  solar  zenith  angle. 
Near  the  subsolar  region,  the  ionopause  was  at 
250  to  280  km  (155  to  174  miles).  With  an 
increase  in  the  Sun  zenith  angle  x,  the  iono- 
pause height  increased.  This  dependence  had 
the  following  form:  l/cos2x.  In  other  words,  it 
corresponded  to  variations  with  zenith  angle 
of  the  solar  wind's  dynamic  pressure  pv2  cos2* 
(p  is  density  and  v  velocity  of  the  solar  wind). 

In  the  stagnation  region,  where  cos2*  =  1  and 
the  dynamic  pressure  is  maximum,  the 
ionopause  is  much  nearer  the  surface.  At  the 
flanks,  with  an  increase  in  x,  it  moves  farther 
away  from  the  surface  and  experiences  greater 
variations  in  height.  Beginning  with  a  zenith 
angle  of  approximately  58°  to  60°,  a  region 

appeared  above  the  main  ionization  maxi- 
mum. This  region  had  an  almost  constant 
electron  number  density  on  the  order  of 
103  cm-3.  It  also  displayed  an  extension  of 
roughly  300  km  (186  miles)  or  more,  the 
so-called  "ionosheath."  The  Pioneer  Venus 
data  showed  that  heights  of  the  upper  iono- 
spheric boundary  vary  considerably.  The 
amplitude  of  its  variations  increased  with 
zenith  angle,  but  the  character  of  the  bound- 
ary behavior  was  generally  the  same  as  that 
shown  by  Veneras  9  and  10  data.  The  large 
range  in  ionopause  heights  that  Pioneer  Venus 
measured  was  due  to  differences  in  measure- 
ment techniques.  Data  that  gave  the  positions 
were  from  various  sensors  that  were  subjected 
to  the  effect  of  the  vehicle  potential,  especially 
near  the  terminator.  During  transfer  from  the 
illuminated  to  nonilluminated  portion  of  an 
orbit,  the  photocurrent  from  the  vehicle 
decreases  in  the  shadow.  Consequently,  the 
potential  of  the  free  body  in  the  plasma 
decreases,  which  affects  the  zero  reference  in 
measurements  with  traps. 

Another  reason  might  be  that  the  very  low 
position  of  its  periapsis  may  have  caused  the 
Pioneer  Venus  trajectory  in  the  ionosphere  to 
give  a  horizontal  rather  than  vertical  cross 
section.  The  results  then  would  depend  on 
horizontal  plasma  variations,  which  perhaps 
were  even  greater  than  usually  appear  in 
radio-occultation  data. 

In  any  case,  according  to  radio-occultation 
observations  on  Pioneer  Venus  and  Veneras  9 
and  10,  these  ionopause  variations  were  less 
striking.  However,  this  problem  required 
further  analysis  and  correlation. 

With  increasing  distance  from  the  subsolar 
point,  the  boundary  between  the  solar  wind 
and  the  ionosphere  becomes  unstable.  The 
magnetohydrodynamic  boundary  layer 


develops  because  of  viscous  interaction  of  two 
plasmas,  instabilities,  and  dissipation  of  energy. 
Its  thickness  grows  to  the  flanks.  Possibly 
the  ionosheath  formation  on  the  electron 
number  density  profile  is  associated,  in  a  yet 
unknown  way,  with  the  formation  of  this 
boundary  layer. 

How  much  solar  wind  penetrates  to  Venus' 
ionosphere?  Is  it  30%  of  the  flux  coming 
toward  the  planet,  or  is  it  less? 

Based  on  indirect  data  (T.  Breus,  Space 
Research  Institute)  and  theoretical  estimates 
(P.  Cloutier  and  R.  Danniel),  the  absorption 
should  be  negligibly  small.  Actually,  it  should 
not  exceed  1%  because  the  shock  front 
position  near  Venus  is  sufficient  to  follow  the 
law  of  magnetohydrodynamic  flow  around  an 
impenetrable  obstacle.  Pioneer  Venus  results 
later  confirmed  this  value. 

Venus'  Nightside  Ionosphere 
It  became  evident  after  radio-occultation 
experiments  onboard  Mariners  5  and  10  and 
Veneras  9  and  10  that  Venus'  nightside 
ionosphere  was  irregular.  Electron  density 
profiles  in  the  nightside  ionosphere  sometimes 
had  two  narrow  maxima  of  roughly  the  same 
order  of  magnitude.  These  maxima  were  5  to 
10  km  (3  to  6  miles)  apart.  Sometimes  the 
number  density  in  the  upper  maximum 
exceeded  that  in  the  lower  one.  It  was  natural 
to  associate  irregular  electron  density  varia- 
tions in  the  nightside  ionosphere  with  the 
influence  of  solar-wind  flows.  It  was  just  such 
an  assumption  that  Soviet  and  American 
specialists  made  after  their  respective  Venera  4 
(1967)  and  Mariner  5  experiments.  But  it  was 
still  obscure  how  the  solar  wind  penetrated  to 
such  low  heights  in  regions  far  from  the  ter- 
minator. (This  was  before  Veneras  9  and  10 
experiments  and  before  discovery  of  the 
plasma  magnetic  tail  near  Venus.)  The 

assumptions  and  estimates  on  how  solar-wind 
electron  fluxes  ionized  Venus'  nightside 
atmosphere  seemed  inconclusive. 

American  researchers  suggested  another 
hypothesis.  They  assumed  that  hydrogen  and 
oxygen  ions  forming  in  the  dayside  iono- 
sphere were  transported  with  the  solar-wind 
flux  to  Venus'  nightside.  The  ions  then 
diffused  down  to  the  heights  of  the  main 
maximum  of  the  night  ionosphere  and 
exchanged  charge  with  neutral  molecules  of 
CO2  and  O2.  As  a  result,  ions  O2+,  O+,  and 
CO2+  formed,  and  the  nightside  ionosphere 
consisted  of  these  ions. 

Veneras  9  and  10  measured  electron  fluxes  at 
an  altitude  of  1500  km  (932  miles)  in  the 
region  of  Venus'  optical  umbra  (see  Figure  7-12). 
K.  Gringauz  and  his  colleagues  Verigin,  Breus, 
and  Gomboshi  suggested  that  these  fluxes  can 
ionize  the  atmosphere  and  form  the  upper 
maximum  of  the  night  ionization. 

Calculations  showed  that,  because  of  these 
electron  fluxes,  the  maximum  of  the  electron 
number  density  could  really  form,  which 
corresponded  to  the  radio-occultation  measure- 
ments of  Veneras  9  and  10  (Figure  7-15).  The 
fact  that  electron  density  variations  in  the  flux 
at  altitudes  of  1500  km  (932  miles)  correlated 
well  with  those  in  the  ionosphere's  upper 
maximum  also  argued  in  favor  of  the  assump- 
tion. The  calculated  and  experimental  profiles, 
however,  coincided  only  when  the  neutral 
atmosphere  density  in  the  calculations  (that  is, 
an  initial  ionizable  material)  was  more  than  an 
order  of  magnitude  less  than  in  available 
models.  The  neutral  temperature  also  might  be 
lower  than  in  these  models.  Veneras  9  and  10 
radio-occultation  measurements  (N.  Savich) 
also  showed  the  neutral  temperature  to  be 
much  lower  (about  100  K)  than  had  been 
suggested  before.  Other  observations  need 

explanations,  too.  For  example,  scientists  knew 
that  electron  fluxes  coming  into  the  atmo- 
sphere caused  nighttime  glows.  Experiments, 
however,  did  not  show  these  glows.  Another 
question  puzzled  scientists:  How  were  elec- 
trons at  1500  km  (932  miles)  able  to  reach 
140  km  (87  miles)? 

An  explanation  is  also  needed  for  the  ioniza- 
tion  source  that  produces  the  second  maximum 
in  the  nightside  ionosphere,  which  frequently 
has  the  same  order  of  magnitude  as  the  upper 
one.  lonization  sources  such  as  ion  transport 
from  the  dayside  ionosphere  and  diffusion  and 
charge-exchange  of  ions  with  atmospheric 
molecules  can  hardly  account  for  one  or  two 
very  narrow  maxima  that  have  been  observed 
in  experiments.  Electrons  with  energies  greater 
than  70  eV,  which  Soviet  scientists  had  used  in 
the  calculations  described  earlier,  could  not 
reach  the  lower  maximum  because  they  "died" 
at  higher  altitudes. 

American  specialists  (D.  Butler  and  J.  Cham- 
berlain) and  a  Soviet  specialist  (V.  Krasnopolsky) 
hypothesized  that  the  lower  maximum  formed 
as  a  result  of  meteor  ionization  at  an  altitude 
level  where  the  number  density  of  neutrals  was 
1012  to  1013  cm-3.  This  level  was  actually  lower 
by  about  20  km  (12.5  miles)  than  that  for 
2xl09  cm-3,  at  which  the  upper  ionization 
maximum  that  K.  Gringauz  and  his  colleagues 
had  estimated  is  formed.  Meteor  ionization 
could  produce  a  rather  narrow  maximum. 
Despite  criticism  and  correction  of  the  avail- 
able neutral  atmosphere  models,  Soviet 
investigators  followed  this  hypothesis  based 
on  their  own  data. 

Eventually,  Pioneer  Venus  data  verified  the 
results  of  calculations  that,  in  turn,  confirmed 
this  hypothesis.  These  data  indicated  that  the 
number  density  of  neutral  components  and 
plasma  temperature  at  the  height  of  the 

ionization  upper  maximum  was  several  orders 
of  magnitude  less  than  in  available  models 
(Figure  7-16).  The  neutral  temperature  in 
Venus'  nightside  atmosphere  was  about 
100  to  140  K. 

Pioneer  Venus  detected  fluxes  of  electrons 
with  energies  less  than  or  equal  to  250  eV  (the 
upper  threshold  of  the  instruments)  at  an 
altitude  of  140  km  (87  miles).  The  intensity  of 
the  flux  was  sufficient  to  produce  ionization 
equal  to  that  measured  experimentally.  This 
information  was  conclusive  evidence  that  the 
Soviet  hypothesis  for  an  electron  source  of 
ionization  in  Venus'  upper  ionosphere 
was  correct. 

Pioneer  Venus  measured  velocities  of  the 
O+  ion  transport  from  the  dayside  to  the  night- 
side  ionosphere.  These  velocities  were  suffi- 
cient to  sustain  the  nightside  ionosphere. 
However,  the  maximum  of  the  ionization  so 
formed  gradually  decreased  with  increasing 
height  in  the  region  above  the  maximum. 
Soviet  data  showed  that  the  thickness  of  the 
ionization  layer  at  the  maximum  half-width 
level  exceeded  by  about  two  times  the 
thickness  of  the  experimental  profile  layer. 

From  these  observations,  it  became  clear  that 
electron  fluxes  help  form  the  narrow  upper 
maximum  of  ionization  in  the  planet's  night- 
side  ionosphere.  It  is  even  possible  that 
double-component  electron  flux  (consisting  of 
electrons  with  energy  less  than  70  eV  and 
greater  than  350  eV)  forms  double  maxima  of 
very  irregular  ionization.  It  also  is  possible  that 
accelerated  fluxes  of  ions  that  Veneras  9  and 
10  detected  in  the  tail  form  the  lower  maxi- 
mum (T.  Breus,  A.  Volacitin,  and  H.  Mishin). 
The  transport  of  O  ions  from  the  dayside 
ionosphere  contributes  mainly  to  the  forma- 
tion of  the  ionosphere's  upper  region. 


Figure  7-15.  Comparison  of 
the  electron-number  density 
profile  in  Venus'  nightside 
ionosphere  (from  Venera  7  0 
data  that  measured  electrons 
ionizing  the  atmosphere)  with 
the  profile  obtained  by  radio- 
occultation  measurements. 



I    140 



Venera  9 
October  28,  1975 

Calculated  profile 

Radio  occupation 



.8  1.0 







Where  do  electron  fluxes  appearing  in  the 
planet's  optical  umbra  form?  How  do  they  enter 
the  atmosphere  at  altitudes  of  100  to  140  km 
(62  to  87  miles)? 

Veneras  9  and  10  detected  a  plasma-magnetic 
tail  near  Venus.  This  discovery  provides  at 
least  a  partial  answer  to  these  questions.  For 
the  present,  it  allows  appropriate  assumptions 
to  be  made. 

Indeed,  in  the  plasma  sheath,  acceleration  of 
solar-wind  particles  was  observed,  the  latter 
flowing  into  the  tail  from  its  flanks.  Also, 
acceleration  of  ions  and  electrons  in  the  day- 
side  ionosphere  could  occur  and  these  could 
be  transported  to  the  tail  and  picked  up  by  the 
solar-wind  flux. 

Different  mechanisms  in  the  tail  can  accelerate 
electron  fluxes.  These  fluxes  can  precipitate 
and  then  be  injected  into  the  atmosphere  at 
low  altitudes  to  produce  an  irregular  source  of 
ionization.  Such  a  source  essentially  depends 
on  the  properties  of  the  solar-wind  and  the 
situation  in  interplanetary  space. 

The  plasma  and  magnetic  experiments  the 
Soviets  conducted  near  Venus  for  over  a 
decade  were  very  useful.  At  the  XVII  General 
Assembly  of  the  International  Association  of 

Geomagnetism  and  Aeronomy  in  Canberra, 
Australia  (December  1979),  results  of  magnetic 
and  plasma  measurements  near  Venus  were 
summarized.  Here  is  a  list  of  basic  results 
obtained  by  Soviet  (Veneras  9  and  10)  and 
American  (Pioneer  Venus)  investigators.  The 
list  also  includes  theoretical  work  and  models 
that  contributed  much  to  the  interpretation  of 
the  results: 

•  Discovery  of  the  plasma-magnetic  tail 
(Venera  vehicles) 

•  Identification  of  the  induced  nature  of  the 
magnetic  field  measured  near  Venus  (Venera 
vehicles  and  Pioneer  Venus) 

•  Determination  of  the  shock  front  position 
(Venera  vehicles) 

•  Detection  of  the  shock  front  asymmetry 
(Venera  vehicles) 

•  Hypothesis  of  an  electron  source  of  night- 
side  ionosphere  ionization  (Venera  results 
and  calculations) 

•  Confirmation  of  the  Venus  "induced"  tail  in 
laboratory  simulation  experiments 
(Soviet  data) 

•  Evidence  for  the  pressure  balance  at  the  iono- 
pause,  sustained  by  the  "magnetic  barrier" 
and  the  ionosphere  thermal  plasma  pressure 










:  Marov 



•     Niemann  et  al. 

V     \ 

\  \ 

\  \ 

Gringauz  et  al.  \ 

(1977)     \      '\ 


Dickinson  and 
Ridley  (1977) 











Figure  7- 1 6.  Dependence 
on  a  height,  h,  of  number 
density  of  neutral  particles, 
nn,  according  to  the  models 
by  M.  Marov  and  O.  Rjabov 
(Institute  of  Applied  Mathe- 
matics, U.S.S.R.  Academy  of 
Sciences),  R.  Dickinson,  and 
E.  Ridley.  The  dependence  nn  (h), 
suggested  by  the  group  headed 
by  K.  Gringauz  (Space  Research 
Institute,  U.S.S.R.  Academy  of 
Sciences),  agrees  with  the 
results  of  H.  Niemann  et  al. 
obtained  from  Pioneer  Venus. 

on  the  one  hand  and  by  solar-wind  stream- 
ing pressure  on  the  other  (Pioneer  Venus) 

•  Discovery  of  magnetic  "flux  ropes"  in  the 
ionosphere  (Pioneer  Venus) 

•  Explanation  of  the  nature  of  the  magnetic 
flux  ropes  (Soviet  and  American  interpre- 
tation of  results) 

•  Detection  of  the  magnetic  field  increase 
before  the  ionopause  in  laboratory  and 

numerical  experiments,  confirming  the 
existence  of  the  magnetic  barrier  (Soviet 

Prospects  for  Further  Research 

Not  everything  we  have  learned  about  Venus 
appears  here.  Our  knowledge  of  the  planet  has 
been  enriched  considerably.  But  has  Venus 
ceased  to  be  a  mystery  planet?  Unfortunately 
(or  fortunately),  the  answer  is  no.  Venus  still 
has  many  mysteries.  While  earlier  puzzles  were 



unraveled  and  many  problems  were  solved, 
new  mysteries  arose  which  are  much  more 
difficult  to  understand. 

Some  of  the  problems  yet  to  be  solved  are: 

•  We  still  have  no  true  explanation  for  the 
higher  content  of  primordial  inert  gases 
on  Venus. 

•  It  is  entirely  unclear  why  there  is  so  little 
water  in  the  Venusian  atmosphere.  Has 
Venus  formed  without  water?  Is  water  hid- 
den in  the  crust,  or  was  it  lost  during  the 
planet's  evolution?  Why  is  the  vertical 
profile  of  water  vapor  concentration  so 

•  We  have  not  yet  determined  the  chemical 
composition  of  the  cloud  cover  particles. 

•  We  do  not  understand  the  mechanism 
responsible  for  the  motion  of  the  atmo- 
sphere at  altitudes  of  40  to  70  km 

(25  to  43  miles),  the  four-day  rotation. 

•  How  active  is  the  planet's  interior?  Is  there 
volcanic  or  seismic  activity? 

•  Finally,  we  do  not  know  when  the  present 
temperature  conditions  of  Venus'  atmo- 
sphere and  surface  set  in.  Did  these  condi- 
tions exist  when  Venus  formed?  Or  was 
Venus'  climate  more  moderate  during  a 
sufficiently  long  initial  epoch? 

How  should  the  exploration  of  Venus  con- 
tinue? Evidently,  only  spacecraft  of  different 
types  can  solve  such  diverse  problems.  To 
study  atmosphere  dynamics,  balloons  are 
indispensable.  We  also  could  use  them  to 
investigate  the  cloud  cover's  physical  and 
chemical  properties. 

Descenders,  or  probes,  are  needed  to  study  the 
chemistry  of  the  minor  constituents  of  Venus' 
atmosphere  and  its  thermal  budget.  These 

spacecraft  would  operate  along  the  usual 
descent  trajectory  from  parachute  deployment 
to  touchdown.  For  best  results,  they  should 
begin  to  function  at  the  highest  altitude 
possible,  at  no  less  than  70  km  (43  miles). 
Finally,  seismic  observations  require  that 
instruments  remain  on  the  planet's  surface  for 
many  months.  Engineers  must  design  this 
special  equipment  to  operate  at  high  tempera- 
tures. The  technical  problems  are  numerous, 
but  we  are  hopeful  that  we  can  solve  them.  We 
also  expect  that  new  and  more  sophisticated 
instruments  will  appear. 

Another  interesting  program  was  the  Soviet- 
French  Vega  project.  This  program  included 
two  new  spacecraft  that  were  improvements 
on  Veneras  11  and  12.  These  spacecraft  would 
fly  by  the  planet  and  jettison  two  landers  for 
a  soft  landing  on  the  planet.  Each  flyby  also 
would  inject  two  balloons  to  study  atmo- 
spheric dynamics. 

The  remaining  Russian  contribution  to  this  book 
(below)  refers  to  the  Vega  mission.  The  new 
spacecraft's  mission  to  Venus  and  to  Halley's 
comet  was  highly  successful.  Its  results  appear  in 
the  next  section  of  this  chapter. 

The  Vega  landers  are  designed  to  study 
chemical  composition  of  inert  gases,  aerosol 
particles,  thunderstorms,  and  other  properties 
during  their  descent.  These  landers  are 
equipped  to  measure  pressure,  temperature, 
chemical  composition  of  Venus'  soil,  and 
possibly  seismic  activity. 

A  particularly  fascinating  Vega  mission 
involved  one  of  the  brightest  and  most 
interesting  comets  in  the  Solar  System.  The 
comet  Halley  approaches  the  Sun  once  every 
76  years.  Such  an  event  occurred  in  1986,  and 
Soviet  scientists  prepared  a  Vega  mission  to 
record  the  event. 

Comets  can  help  us  understand  the  Universe's 
origin  and  evolution.  There  is  an  assumption 
that  comet  nuclei  are  the  material  from  which 
the  planetary  system  formed.  Until  the  Vega 
mission,  astronomers  could  only  study  comets 
with  ground-based  instruments.  We  knew  prac- 
tically nothing  about  the  structure  of  comets' 
nuclei,  ionization  sources,  mechanisms  for 
formation  of  plasma  structures  in  comets'  tails, 
and  the  reasons  for  the  comets'  various  shapes. 

Conditions  for  observing  the  comet  from  Earth 
were  relatively  unfavorable  in  1986.  So  study- 
ing Halley's  comet  from  space  was  particularly 
important.  To  investigate  Halley's  comet,  the 
European  Space  Agency  launched  the  comet 
flyby  spacecraft  Giotto.  Japan  launched  two 
spacecraft,  Sakigake  and  Suisei. 

The  Soviets  had  not  planned  a  special  mission 
to  the  comet.  However,  Vega  flyby  vehicles  to 
Venus  were  able  to  use  a  gravitational  maneu- 
ver near  the  planet  to  travel  on  to  the  comet 
(Figure  7-17).  These  vehicles  approached  within 
several  thousand  kilometers  of  the  comet  and 
were  able  to  photograph  its  nucleus.  Among 
many  other  phenomena,  they  studied  compo- 
nents of  the  dust  and  gas  that  evaporated  from 
the  nucleus,  and  ion  concentrations.  These 
three  projects — European,  Japanese,  and 
Soviet — complemented  each  other,  in  terms  of 
both  scientific  goals  and  equipment. 


R.  O.  Fimmel,  L.  Colin,  E.  Burgess 

We  have  added  the  following  material  to  this 
chapter  to  give  you  information  that  goes  beyond 
the  period  covered  by  the  Soviet  authors.  This 
material  documents  other  events  in  the  exploration 
of  Venus.  It  covers  the  period  from  the  extended 
Pioneer  Venus  mission  to  the  beginning  of  NASA's 
Magellan  mission  to  Venus. 

After  missing  the  1976-1978  launch  opportu- 
nity, the  Soviets  sent  their  next  mission  to 
Venus  in  September  1978.  Veneras  11  and  12 
were  each  a  combination  flyby  and  lander 
spacecraft.  They  arrived  in  December  1978. 
The  flyby  spacecraft  gathered  data  on  the 
ultraviolet  spectrum  of  the  upper  atmosphere 
as  they  sped  by  Venus.  They  successfully 
telemetered  these  data  back  to  Earth. 

Both  landers  provided  atmospheric  data  as 
they  penetrated  the  atmosphere  before  land- 
ing safely  on  the  surface.  During  the  descent 
through  the  atmosphere,  an  instrument 
designed  to  search  for  "thunderstorm"  activity 
recorded  radio  bursts  that  might  be  attributed 
to  lightning.  These  data  reached  Earth  about 
five  days  before  Fred  Scarf  detected  "whistlers" 
with  the  Pioneer  Venus  instruments  (see 
Chapter  6).  Pioneer's  orbital  configuration  did 
not  allow  an  earlier  search  for  such  whistlers. 

The  Venera  landers  found  the  ratio  of  argon-40 
to  argon-36  was  several  hundred  times  less 
than  in  Earth's  atmosphere.  Why  did  Venus 
have  so  little  argon-40,  a  decay  product  of 
potassium-40?  The  amount  of  this  potassium 
isotope  in  Venusian  rocks  is  about  the  same  as 
in  terrestrial  rocks.  One  possibility  is  that 
Venus  may  not  have  experienced  as  much 
volcanic  activity  as  Earth.  Arguing  against  this, 
however,  are  the  images  returned  by  Magellan 
showing  that  Venus  has  experienced  a  great 
deal  of  volcanic  activity. 

The  issue  might  be  resolved  if  atmospheres 
were  the  result  of  comet  impacts  rather  than 
mainly  the  result  of  internal  activity  and 
evolution  of  volatiles  from  within  the  planets. 
In  such  a  scenario,  incoming  material,  not 
planetary  material,  would  govern  isotopic 
ratios.  The  atmospheric  ratios  would  bear  no 
relationship  to  ratios  in  the  material  of  the 
planet  itself. 


Figure  7- 1 7.  Vega-Halley 
mission — the  red  line  shows  the 
flight  trajectory  of  Halley's 
comet;  the  yellow  line  shows  the 
trajectory  of  the  flyby  vehicles. 
The  green  and  blue  lines  show 
the  orbit  of  Venus  and  Earth, 
respectively.  Top  left  shows  the 
descent  module  separating  from 
the  flyby  spacecraft  at  Venus.  At 
the  top  right  is  a  general  view  of 
a  prototype  of  the  Vega  space- 
craft: 1)  the  flyby  spacecraft, 
and  2)  the  descent  module.  The 
final  version  carried  a  balloon 
probe  and  a  lander  in  the 
descent  module.  In  bottom  right, 
the  flyby  spacecraft  is  passing 
by  Comet  Halley,  almost  nine 
months  after  its  encounter 
with  Venus. 


Venera  12's  lander  settled  on  the  surface  near 
Phoebe  Regio,  where  It  measured  a  surface 
temperature  of  480°C  (896°F).  It  also  recorded 
an  atmospheric  pressure  88  times  greater  than 
Earth's  sea  level  pressure.  The  flyby  spacecraft 
relayed  telemetry  from  the  lander.  After 
roughly  110  minutes  of  data  relaying,  the  flyby 

spacecraft  went  below  the  horizon  as  viewed 
from  the  landing  site.  Communication  ended. 
The  other  lander,  Venera  11,  measured  close  to 
the  same  atmospheric  pressure,  but  it  recorded 
a  temperature  some  34°C  (93°F)  less  than  at 
the  Venera  12  site  about  725  km  (450  miles) 
farther  north  in  Phoebe  Regio.  This  lander  lost 

contact  with  the  flyby  spacecraft  95  minutes 
after  landing. 

No  Russian  spacecraft  were  sent  to  Venus  at 
the  1980  opportunity.  The  next  missions  were 
Veneras  13  and  14,  two  spacecraft  launched 
on  October  30  and  November  4,  1981  (see 
Table  7-3).  Again  the  buses  that  transported 
the  landers  to  Venus  were  flybys.  This  arrange- 
ment allowed  more  weight  to  be  allocated  to 
the  landers  for  a  given  launch  weight.  Soviet 
scientists  again  targeted  the  landers  for  the 
Phoebe  Regio  area,  where  they  landed  on 
March  1  and  March  5,  1982,  respectively. 

These  landers  accomplished  the  high  technol- 
ogy task  of  gathering  small  soil  samples  from 
the  planet's  hot  surface  and  examining  them 
without  exposing  the  interior  of  the  landers  to 
the  high  surface  pressure  and  temperature.  The 
samples  were  analyzed  within  closed  cham- 
bers. A  series  of  airlocks  moved  the  samples  by 
airstream  to  chambers  of  decreasing  pressures. 
The  last  chamber  had  a  temperature  of  only 
30°C  (86°F)  and  a  pressure  one-tenth  of  Earth's 
at  sea  level.  Basaltic  sand  and  dust  was  identi- 
fied in  the  samples  at  both  landing  sites.  Of 
the  two  varieties  in  the  samples,  one  is  scarce 
on  Earth.  No  granite  was  found  in  the  sample 
material.  The  landers  had  improved  photo- 
imaging  systems  and  returned  excellent  images 
to  Earth  for  color  reproduction.  These  showed 
the  typical  flat  Venusian  landscape  strewn 
with  flattened  rocks  and  weathered  lava  flows. 
Fine  rubble  covered  much  of  the  surface  at  the 
Venera  13  site,  but  there  was  much  less  of  this 
material  at  the  Venera  14  site,  which  was 
725  km  (450  miles)  farther  south  and  740  km 
(460  miles)  to  the  west  in  a  lower  area.  Fluid 
lavas  from  the  Phoebe  volcanoes  may  have 
covered  this  area.  Plate-like  rocks  visible  in  the 
Venera  14  and  other  surface  images  so  resemble 
sedimentary  rocks  that  some  Russian  experi- 
menters have  suggested  that  Venus'  high 

atmospheric  pressures  may  have  led  to  a 
sedimentation  process  in  the  atmosphere 
similar  to  that  in  bodies  of  water  on  Earth. 

Dust  suspended  in  the  atmosphere  colored 
the  sky  orange,  somewhat  like  the  skies  of 
Mars.  The  light  yellowish-orange  color  of  soil 
and  rocks  suggests  that  the  surface  is  heavily 
oxidized,  again  like  Mars.  This  oxidation 
might  imply  that  the  planet  once  possessed 
large  bodies  of  water,  the  oxygen  from  which 
became  trapped  in  surface  rocks  while 
hydrogen  escaped  into  space. 

Venera  14's  instruments  also  detected  what 
scientists  believed  were  slight  seismic  distur- 
bances. However,  the  other  lander  did  not 
detect  any  such  disturbances. 

Veneras  13  and  14  temperature  and  atmo- 
spheric pressure  readings  were  very  similar  to 
earlier  lander  measurements.  These  landers 
operated  successfully  for  much  longer  than 
their  design  lifetimes.  Venera  13  survived  for 
127  minutes.  Venera  14  lasted  for  63  minutes 
at  a  lower  elevation  and  at  a  pressure  of  nearly 
100  atmospheres. 

Soviet  scientists  launched  Veneras  15  and  16 
on  June  2  and  June  6,  1982.  These  spacecraft 
went  into  near  polar  orbit  around  Venus  in 
October  1982.  Like  Pioneer  Venus  Orbiter, 
they  had  an  orbit  with  a  period  of  24  hours. 
Periapsis  was  a  few  thousand  kilometers  over 
the  northern  hemisphere  and  apoapsis  about 
65,000  km  (40,391  miles)  above  the  southern 
hemisphere.  Science  data  began  flowing  to 
Earth  in  late  October.  The  spacecraft  carried 
advanced  side-looking  radar,  which  produced 
surface  radar  maps  at  higher  resolution  than 
those  from  Pioneer  Venus  Orbiter.  The 
spacecraft  also  were  able  to  map  the  surface 
into  higher  northern  and  southern  latitudes, 
supplementing  the  Pioneer  data.  A  radar  map 


Table  7-3.  Soviet  Space  Vehicles  That  Studied  Venus  1979  to  1986 

Space  vehicle 






Venera  1  3 




Landed  in  region  of  Phoebe  Regio;  analyzed 


soil  samples;  returned  colored  photo  images 

from  surface 

Venera  14 


1  1  /4/81 


Same  as  Venera  14  but  from  another  site 


Venera  1  5 




Radar  maps  of  the  planet  to  high  northern 

and  southern  latitudes 

Venera  1  6 




Same  as  Venera  14 





Lander  continued  work  of  earlier  landers,  but 

in  Aphrodite  Terra  region. 


Probe  carried  by  balloon  around  planet 

sampling  atmosphere. 


Flyby  spacecraft  continued  to  a  flyby  of 

Halley's  comet  on  3/6/86 

Vega  2 



1  /1  5/85 

Same  as  for  Vega  1 

Probe  Flyby 


Encounter  3/9/86 


atlas  was  published  confirming  the  earlier 
Pioneer  Venus  Orbiter  maps,  which  showed 
Venus  as  a  very  complex  planet.  The  Soviet 
maps  revealed  many  shield  volcanoes,  volcanic 
domes,  lava  flows,  chaotic  terrain,  long  ridges, 
great  depressions,  parallel  mountains  and 
valleys,  regions  of  intersecting  ridges  and 
valleys,  and  many  impact  craters.  Later,  all 
these  features  appeared  with  greater  detail  in 
NASA  Magellan  images.  Planetologists  inter- 
preted some  features  as  evidence  of  plate  tec- 
tonics and  earthquake-type  displacements.  A 
major  surprise  was  that  erosion  has  degraded 
many  craters.  The  spacecraft  also  used  infrared 
spectrometers  to  map  Venus.  These  images 
showed  warmer  temperatures  at  the  poles  than 
at  the  equator  and  confirmed  that  there  is 
little  difference  between  day  and  night 

The  New  Vega  Spacecraft 

After  the  Venera  series,  the  Soviet  Union 
continued  Venus  exploration  with  a  new  type 
of  spacecraft,  Vega.  These  advanced  spacecraft 
consisted  of  a  flyby  bus,  an  atmospheric 

balloon  probe,  and  a  soft  lander.  The  balloon 
probes  each  carried  four  scientific  experiments, 
the  landers  carried  nine.  The  multipurpose  bus 
carried  the  probe  and  lander  to  Venus  and 
then  continued  to  a  rendezvous  flyby  with 
Comet  Halley. 

The  mission  started  in  December  1984  with 
launches  of  two  identical  spacecraft,  Vega  1 
and  Vega  2.  Both  arrived  at  Venus  in  June  1985. 
Explosive  bolts  fired,  and  the  descent  capsules 
separated  from  the  flyby  bus.  The  Vega  1 
descent  capsule  entered  the  atmosphere  on 
June  11,  1985;  Vega  2  entered  on  June  15. 

At  about  125  km  (78  miles)  above  the  surface, 
each  descender  separated  into  a  lander  and  a 
balloon-sonde.  The  landers  were  targeted  for 
the  area  north  of  Aphrodite  Terra,  where  they 
soft-landed  successfully.  The  balloon-sondes 
used  a  French-designed  balloon  and  drifted 
through  the  atmosphere  for  about  two  Earth 
days  at  an  altitude  of  about  50  km  (31  miles). 
Venusian  winds  carried  them  along  at  an 
average  speed  of  about  65  m/sec  (about 

146  mph).  As  each  balloon  traveled  some 
10,000  km  (6214  miles)  around  Venus,  an 
international  team  of  scientists  evaluated  their 
paths.  Instruments  recorded  changes  in  light 
intensity  but  produced  no  conclusive  evidence 
of  lightning  flashes. 

The  landers  continued  the  Veneras'  earlier  work 
with  more  advanced  instrumentation.  Soil 
analysis  by  the  Vegas  discovered  anorthosite- 
troctolite,  a  rock  that  is  quite  rare  on  Earth  but 
common  on  the  primitive  crusts  of  the  Moon 
and  Mars.  As  they  descended  through  the 
atmosphere,  the  landers  also  used  instruments 
to  sample  the  clouds  to  determine  their 
sulfuric  acid  content.  Both  landers  provided 
gas  chromatograph  and  chemical  reactor  data 
that  showed  that  clouds  between  48  and  63  km 
(30  and  39  miles)  contained  one  milligram  of 
sulfuric  acid  per  cubic  meter.  A  mass  spectrom- 
eter and  an  aerosol  collector  confirmed  these 

Small  samples  of  clouds  were  excited  by 
x-ray  to  reveal  the  presence  of  other  compo- 
nents. These  included  elemental  sulfur, 
chlorine,  and  phosphorous.  Other  instruments 
determined  the  way  in  which  light  is  diffused  by 
the  clouds  and  discovered  that  the  particles 
have  a  size  of  about  a  tenth  of  a  micron.  (One 
micron  is  one  thousandth  of  a  millimeter.) 
The  particle  size  determinations  differed  from 
the  trimodal  distribution  that  Pioneer  Venus 
probes  (1978)  and  Venera  probes  measured. 
The  spacecraft  identified  two  cloud  layers  at 
50  and  58  km  (31  and  36  miles)  above  the 
mean  surface  and  each  was  about  5  km 
(3  miles)  thick.  The  cloud  layer  results  differed 
from  earlier  Soviet  probes,  suggesting  that 
major  changes  occur  in  the  clouds  over  large 
regions,  since  the  two  Vega  probes  entered  the 
atmosphere  some  1500  km  (932  miles)  from 
each  other. 

Comet  Halley 

In  1986,  the  flyby  Vega  spacecraft  hurtled  past 
Comet  Halley  on  March  6  and  March  9.  Each 
spacecraft  carried  15  scientific  instruments  for 
an  international  group  of  experimenters  from 
nine  countries.  The  first  objective  was  to 
obtain  a  good  look  at  the  nucleus,  which 
appears  only  as  a  star-like  body  from  Earth. 

The  spacecraft  observed  the  nucleus  from  dis- 
tances of  8000  to  9000  km  (4971  to  5593  miles). 
It  was  an  elongated  body  some  14  km 
(8.7  miles)  long  and  7.5  km  (4.7  miles)  across, 
somewhat  curved  and  irregular  but  definitely 
not  two  bodies.  Images  from  the  two  space- 
craft suggested  a  53-hour  rotation  period  for 
the  nucleus.  Even  though  dust  clouds  obscured 
the  surface,  the  spacecraft  were  able  to  mea- 
sure its  reflectivity  as  being  somewhat  like  that 
of  the  lunar  surface. 

Infrared  measurements  of  the  region  near  the 
nucleus  suggested  a  surface  temperature  higher 
than  predicted  by  the  icy  nucleus  hypothesis. 
Scientists  thought  this  high  temperature  might 
result  from  dust  clouds  close  to  the  surface 
rather  than  from  the  surface  itself.  Ices  must  be 
present  in  the  nucleus  to  give  rise  to  the  gas  in 
the  comet's  coma,  and  evaporation  of  these 
ices  would  be  expected  to  cool  the  nucleus. 
One  explanation  Russian  experimenters  sug- 
gested is  that  the  nucleus'  surface  is  covered 
with  a  thin  refractory  porous  material.  Solar 
radiation  heats  the  top  surface  while  the 
bottom  is  insulated  and  in  contact  with  the  icy 
material.  Heat  can  be  conducted  internally  to 
evaporate  the  ice  while  the  resulting  gases  can 
escape  through  the  porous  material. 

The  spacecraft  also  made  major  contributions 
to  studies  of  the  comet's  dust  cloud.  Scientists 
obtained  hundreds  of  spectra  from  which  they 
determined  the  dust's  composition.  Inter- 
action of  the  comet  with  the  solar  wind  was 


investigated  using  another  group  of  instru- 
ments, which  also  recorded  crossings  of  the 
comet's  bow  shock.  The  information  these 
instruments  gathered  is  important  to  continu- 
ing studies  of  how  Venus  and  other  planetary 
bodies  interact  with  the  solar  wind. 

Studies  of  Venus,  the  other  planets,  and  the 
comets  in  our  Solar  System  will  provide  the  key 
to  a  better  understanding  of  Earth's  evolution. 
Answering  these  questions  is  vitally  important 
to  our  future,  and  efforts  invested  in  such 
projects  are  certain  to  bear  fruit. 






Contributed  by  Thomas  M.  Donahue 
(Professor  of  Planetary  Science, 
University  of  Michigan,  and  Interdis- 
ciplinary Scientist  for  Pioneer  Venus 

In  the  early  years  of  the  space  program, 
NASA  and  the  planetary  science  commu- 
nity were  enthusiastically  selling  missions 
like  Pioneer  Venus  to  members  of 
Congress  and  the  media.  At  that  time,  it 
was  fashionable  to  argue  that  studying 
the  other  planets  would  help  us 
understand  Earth.  There  is  certainly  a 
sense  in  which  this  is  true — and 
results  from  the  Pioneer  Venus 
mission  demonstrate  it  well. 
However,  there  also  is  a  sense  in 
which  it  can  be  misleading.  The 
best  way,  by  far,  to  understand 
Earth's  climate,  weather,  or  any 
of  the  ways  in  which  the.; ' 
planet  works  is  to  study  Earth. 
But  to  understand  Earth  as 
one  member  of  the  collec- 
tion of  planets  in  the  Solar 
System  and,  thus,  to 
understand  how  that 
system  formed  and 
evolved,  we  must  study  more' 
than  Earth.  It  is  necessary  to  study  all  the 
planets  in  detail.  It  would  never  have  done  to 
study  Earth  close  at  hand  and  the  other 
planets  only  with  telescopes.  The  results  that 
Pioneer  Venus  achieved  provide  excellent 
examples  of  why  this  is  true. 

Perhaps  the  outstanding  example  was  the 
enlightenment  the  mission  brought  to  why 
Venus  and  Earth  evolved  into  such  different 
planets.  Venus  and  Earth  are  among  the  four 
terrestrial  planets.  Mercury  and  Mars  are  the 
other  two.  There  is  good  reason  to  believe  that 
the  four  terrestrial  planets  have  the  same 
mixture  of  rocks,  minerals,  and  volatile 
substances  such  as  water  and  nitrogen.  We 
base  these  beliefs  on  numerous  observations  of 
young  solar  systems  elsewhere  in  the  galaxy 
and  sophisticated  computer  simulations.  These 
terrestrial  planets  are  the  final  products  of  the 
coalescence  of  a  great  swarm  of  objects  called 
planetesimals.  These  grew  by  colliding  with 
each  other  until,  after  100  million  years,  only 
four  planets  (plus  the  asteroids)  were  left.  At 
the  end  of  this  accretional  epoch,  a  rain  of 
comet-like  objects  from  the  outer  Solar  System 
may  have  coated  these  planets  with  volatile 
substances.  But  all  terrestrial  planets  should 
have  shared  more  or  less  the  same  endowment 
of  original  planetary  material.  Why,  then, 
are  Venus  and  Earth,  which  are  such  close 
neighbors  and  so  similar  in  size  and  mass, 
so  different? 

Although  they  emerged  from 
the  same  primordial  nebula 
of  gas  and  dust,  Venus  and 
Earth  today  are  strikingly 
different.  Why?  Scientists 
speculate  that,  at  one  time, 
Venus  may  have  had  an 
abundance  of  water.  What 
caused  it  to  disappear?  Can 
the  processes  that  shaped 
Venus  help  us  interpret 
Earth's  continuing  evolu- 
tion? For  example,  can  we 
learn  about  the  greenhouse 
effect  on  Earth  by  studying 
Venus'  past?  This  chapter 
provides  insights  into  these 
and  other  questions. 



A  close  look  at  the  volatile  inventory  from 
Pioneer  Venus  and  the  Venera  spacecraft  data 
provides  some  clues.  These  data  show  that, 
whereas  on  Earth  carbon  in  the  form  of  carbon 
dioxide  reacted  with  silicate  rocks  to  form 
limestone,  roughly  the  same  amount  of  carbon 
dioxide  exists  as  a  gas  dominating  Venus' 
atmosphere.  The  reason  for  the  difference  is 
water.  Earth  has  copious  quantities  of  water 
necessary  for  the  weathering  of  silicates  lead- 
ing to  carbonate  formation.  The  oceans  also 
play  a  crucial  role  in  limestone  formation. 
Venus,  as  Pioneer  measurements  show,  has  less 
water  by  a  factor  of  250,000  than  Earth.  So  what 
happened  to  the  water  that  should  have  been 
as  abundant  on  the  early  Venus  as  it  was  on 
the  early  Earth? 

The  Pioneer  probe  and  orbiter  measurements 
supplied  an  answer.  When  scientists  studied 
deuterium — the  heavy  form  of  hydrogen — 
they  found  that  deuterium  is  150  times  as 
abundant  relative  to  ordinary  hydrogen  on 
Venus  than  it  is  on  Earth.  Deuterium's  abun- 
dance strongly  indicates  Venus  once  had  much 
more  hydrogen  than  it  now  does.  (Lighter  gases 
escape  more  easily  into  space  from  a  planet's 
gravitational  field.)  In  fact,  analysis  showed 
that  it  must  have  had  at  least  300  times  as 
much.  That  is,  it  had  enough  to  form  a  planet- 
wide  sea.  This  sea  would  have  been  between 
4  and  25  meters  deep  if  the  hydrogen  was  in 
the  form  of  liquid  water. 

Theoretical  studies  of  the  way  hydrogen  can 
escape  from  planets,  which  these  observations 
inspired,  showed  that  the  original  amount  of 
water  may  well  have  been  much  greater.  Some 
scientists  estimate  it  as  much  as  the  equivalent 
of  a  full  terrestrial  ocean.  The  water  might  even 
have  been  liquid  because  the  infant  Sun  was 
only  about  75  percent  as  bright  as  today's  Sun. 

With  such  a  faint  Sun  shining,  Venus'  surface 
temperature  would  have  been  low  enough  for 
a  while,  perhaps,  to  have  allowed  an  ocean  to 
exist.  But  later,  when  the  Sun  grew  brighter, 
the  water  would  have  evaporated,  then  con- 
verted into  hydrogen  and  oxygen  in  the  upper 
atmosphere  by  ultraviolet  radiation  and  the 
hydrogen  would  have  rapidly  escaped.  The 
process  is  called  a  runaway  greenhouse. 

In  the  runaway  greenhouse,  limestone  would 
have  released  carbon  dioxide  gas,  which  would 
have  entered  the  atmosphere.  This  would  have 
resulted  in  the  high  surface  temperatures  on 
the  planet  today.  Thus,  it  is  possible  to  explain 
these  high  temperatures  in  terms  of  a  green- 
house effect  involving  carbon  dioxide.  This 
explanation  lends  credibility  to  the  argument 
that  increasing  carbon  dioxide  in  the  Earth's 
atmosphere  also  will  increase  its  temperature. 
On  the  other  hand,  the  Venus  story  inspired 
scientists  to  test  the  possibility  that  we  might 
induce  a  runaway  greenhouse  on  Earth.  They 
found  that  Earth  orbits  too  far  from  the  Sun 
for  that  to  happen. 

The  lack  of  an  ocean  may  be  relevant  to  under- 
standing another  great  difference  between 
Earth  and  Venus.  Earth  gets  rid  of  its  internal 
heat  by  means  of  convective  motions  in  its 
fluid  interior.  These  convective  motions  drive 
the  Earth's  crustal  plates.  Water  may  help 
provide  the  lubricant  that  allows  some  plates 
to  dive  beneath  others  in  the  planet's  subduc- 
tion  zones.  On  the  other  hand,  Pioneer  Venus 
and  Magellan  mission  radar  found  no  evidence 
of  plate  motions  on  Venus,  certainly  not  on 
the  scale  prevailing  on  Earth.  Instead,  these 
radar  images  clearly  showed  that  at  one  time 
Venus  shed  its  internal  energy  by  volcanic 
activity.  But  even  Venus'  volcanism  seems  to 
have  stopped  a  good  while  ago.  Recent 

Magellan  measurements  indicate  that  Venus 
now  has  a  very  thick,  strong,  and,  therefore, 
dry  crust.  The  same  seems  to  be  true  of  Mars. 
Even  the  interior  of  Venus  may  now  be  dry. 

There  is  another  excellent  example  of  a  class 
of  similarities  and  differences  among  Solar 
System  objects.  This  example  concerns  the 
ionospheres  of  terrestrial  planets  and  the 
interaction  of  the  solar  wind  with  them.  This 
particular  observation  only  became  clear  when 
spacecraft  carried  out  in  situ  observations.  In 
the  development  of  this  understanding, 
Pioneer  Venus  played  a  central  role.  The 
ionospheres  of  Venus  and  Earth  are  very 
different  and  the  solar  wind  interaction  with 
them  even  more  so.  Apart  from  the  difference 
in  basic  atmospheric  constituents,  the  major 
reasons  for  the  dichotomy  are  related  to 
Venus'  slow  retrograde  rotation  and  the 
absence  of  an  intrinsic  magnetic  field.  These 
two  traits  may  well  be  related.  In  the  case  of 
Earth,  the  field  seems  to  be  created  by  an 
internal  dynamo  in  the  electrically  conducting 
core.  However,  we  do  not  completely  under- 
stand the  dynamo  mechanism  for  Earth's 
much-studied  field.  Therefore,  it  is  too  much 
to  expect  that  we  would  understand  the  reason 
for  the  absence  of  a  magnetic  field  on  Venus. 
One  observation  is  certain,  though,  as  Pioneer 
Venus  and  the  Venera  missions  demonstrated 
in  exquisite  detail.  The  solar  wind  pushes  close 
to  Venus'  ionosphere  and  interacts  directly 
with  it.  On  Earth,  it  stands  far  off  because  of 
the  shielding  that  Earth's  field  provides. 

After  Pioneer  Venus  began  to  orbit  Venus, 
experts  formed  another  hypothesis  involving 
the  solar  wind.  They  believed  the  interaction 
of  the  solar  wind  with  Venus'  atmosphere 
would  have  a  direct  analog  in  its  interaction 
with  comets,  which  are  also  intrinsically 

unmagnetized.  But,  when  space  missions 
encountered  comets,  such  as  Giacobini-Zinner 
and  Halley,  these  experts  learned  again  the 
need  for  direct  measurements.  The  ease  with 
which  the  solar  wind  picks  up  ions  from  the 
extended  atmosphere  of  comets  completely 
changed  the  physics  of  the  interaction.  In 
many  ways,  the  analogy  failed  to  hold.  But 
only  after  intensive  in  situ  study  of  the  plasma 
environments  of  these  three  different  objects 
were  scientists  able  to  clearly  understand 
their  natures. 

There  are  other  examples.  Thus,  several  factors 
profoundly  influence  atmospheric  circulation 
on  Earth — in  other  words,  Earth's  weather  and 
climate.  These  factors  include  the  Earth's  rapid 
rotation  on  its  axis,  the  inclination  of  that  axis 
(which  is  responsible  for  the  seasons),  and 
atmospheric  pressure  and  composition.  In 
particular,  water,  the  crucial  volatile  present 
on  Earth,  scarce  on  Venus,  is  essential.  In  the 
case  of  Venus,  tracking  of  the  probes  provided 
enough  information  about  atmospheric 
circulation  to  form  the  basis  for  an  eventual 
understanding.  But  clearly,  we  need  more  data 
on  circulation  in  the  deep  atmosphere  and  the 
planet's  radiation  budget.  Only  with  these  data 
will  we  more  completely  understand  the  dif- 
ferences in  the  climates  of  the  two  largest 
terrestrial  planets. 

The  Pioneer  Venus  Probe  and  Orbiter  missions 
allowed  us  to  make  enormous  strides  in  space 
sciences.  They  advanced  our  understanding  of 
the  way  the  objects  of  the  inner  Solar  System 
work,  how  they  originated,  and  how  they 
evolved.  But  only  more  visits  to  Venus  and  the 
other  members  of  the  Solar  System's  family  of 
planets,  satellites,  comets,  and  asteroids  can 
make  that  understanding  adequately  complete 
and  satisfying. 




PIONEERS  booklet 

>  observes  yet  another 
let  in  between  its 
;rvations  of  Venus 




PLAN  published 

First  Magellan  detailed 
radar  images  of  Venus 

Periapsis  falls  closer  to 
Venus;  reaches  lower 

published;  results  of  PVO 

Periapsis  again 

PVO  gathers  details 
about  lower  ionosphere 
and  atmosphere 

In  October  PVO  ends 
its  1 4-year  mission; 
plunges  into  Venus' 


ets  withdraw  from 

-Iran  declare  truce 

is  use  poison  gas  on 
dish  civilians 

Hurricane  Hugo 

San  Francisco,  Loma  Prieta 

Valdez  oil  spill 

Berlin  Wall  falls 

US  troops  invade  Panama 

Tiananmen  Square 

Iraq  invades  Kuwait 
Germany  re-unites 

John  Major  replaces 
Margaret  Thatcher  as 
British  Prime  Minister 

Oil  prices  soar 

Nelson  Mandela 
released  from  prison 
in  South  Africa 

Soviet  Union  is  dissolved 

Operation  Desert  Storm 
frees  Kuwait 

Mount  Pinatubo  erupts 

US  troops  sent 
to  Somalia 

Serious  destructive 
riots  in  Los  Angeles 

Hurricanes  Andrew 
(Florida)  and 
Iniki  (Hawaii) 

988  1989  1990  1991  1992 


Chronology  of  Exploration  of  Venus  from  Earth  before  the  Pioneer  Venus  Mission 
Date  Event 

684  BC  Ninevah  (Babylon)  tablets  record  observations  of  Venus  made  as  early 

as  3000  BC. 

361  AD  Chinese  annals  record  occupation  of  Venus  by  the  Moon. 

845  Chinese  annals  record  an  observation  of  Venus  passing  through  the  Pleiades. 

1587  Tycho  Brahe  records  an  occultation  of  Venus  by  the  Moon. 

1610  Using  the  newly  invented  telescope  Galileo  discovers  that  Venus  exhibits  phases 

like  the  Moon. 

1639  Horrox  and  Crabtree  are  first  to  observe  a  transit  of  Venus  across  the  face  of 

the  Sun. 

1643  Fontana  claims  that  irregularities  along  the  terminator  of  Venus  are  mountains. 

1666  Cassini  observes  bright  and  dusky  spots  on  Venus  and  claims  Venus  rotates  in  a 

little  more  than  24  hr. 

1716  Halley  records  seeing  Venus  in  daylight. 

1726  Bianchini  claims  that  Venus  rotates  in  24  hr. 

1761  Lomonosov  interprets  optical  effects  observed  during  transit  of  Venus  across  the 

Sun  as  being  due  to  an  atmosphere  on  the  planet. 

1769  Captain  Cook  visits  Tahiti  to  observe  transit  of  Venus.  Solar  parallax  determined 

to  within  a  few  tenths  of  an  arcsecond. 

1788  Schroter  claims  that  his  observations  of  Venus  show  that  the  planet  rotates  on  its 

axis  in  23  hr  28  min. 

1792  Schroter  concludes  that  Venus  has  an  atmosphere  because  the  cusps  of 

the  crescent  phase  extend  beyond  the  geometrical  crescent. 

1807  Wurm  determines  the  diameter  of  the  visible  disc  of  Venus  to  be 

12,293  km  (7639  mi.).  279 

1841  De  Vico  claims,  on  the  basis  of  his  observations,  that  Venus  rotates  in  a  period  of 

23  hr  21  min  on  an  axis  included  53°  to  the  planet's  orbit. 

1887  Stroobant  explains  that  all  the  claims  by  astronomers  of  discovering  a  satellite  of 

Venus  were  merely  observations  of  faint  stars. 

1890  Schiaparelli  concludes  from  his  observations  that  Venus  rotates  in  225  days. 










Lowell  produces  drawings  of  Venus  with  broad  dark  lines  that  are  hazy,  ill- 
defined,  and  nonuniform.  He  concludes  from  his  observations  that  Venus 
rotates  in  the  same  time  that  it  revolves  around  the  Sun,  namely,  225  days. 

St.  John  and  St.  Nicholson,  unable  to  detect  any  water  vapor  in  its  atmosphere, 
suggest  that  Venus  is  a  dry,  dusty  world. 

Lyot  measures  the  polarization  of  sunlight  reflected  from  the  clouds  of  Venus 
and  introduces  a  new  method  of  investigating  the  size  and  nature  of  particles  in 
its  clouds. 

Wright  and  Ross  photograph  Venus  through  ultraviolet  filter. 

Adams  and  Dunham  detect  carbon  dioxide  in  the  atmosphere  of  Venus  with  a 
high-dispersion  spectrograph  on  the  Mount  Wilson  100-in.  telescope. 

Wildt  shows  that  the  high  surface  temperature  of  Venus  could  arise  from  a 
greenhouse  effect  in  an  atmosphere  possessing  a  high  proportion  of 
carbon  dioxide. 

Kuiper  begins  a  long  series  of  experiments  with  low-  to  high-resolution 
spectrographs  to  study  rotational  temperature  of  carbon  dioxide  at  the  cloud  tops 
using  infrared  wavelengths. 

Hoyle  suggests  that  the  Venus  clouds  are  a  photochemical  hydrocarbon  smog. 

Mayer,  McCullough,  and  Slonaker  detect  radio  waves  from  Venus  at  3-cm 
wavelengths,  indicating  that  the  surface  temperature  must  be  very  high,  about 
330°C  (626°F). 

Price  makes  the  first  radar  sounding  of  Venus. 

Boyer  discovers  a  4-day  rotation  period  of  ultraviolet  markings  in  Venus'  clouds. 

Sinton  and  Strong  establish  temperature  of  the  cloud  tops  as  -39°C  (-38.2°F),  by 
infrared  bolometry. 

Dollfus,  using  polarimetry,  determines  pressure  at  the  cloud  tops  as  90  mbar. 

Opik  proposes  that  clouds  are  thick  dust  consisting  of  calcium  and 
magnesium  carbonates. 

Sagan  suggests  that  the  high  temperature  of  Venus'  surface  results  from  a 
greenhouse  effect. 

Pettengill  makes  further  radar  observations  of  Venus  and  determines  the 
astronomical  unit  with  high  precision. 

Kuz'min  and  Clarke  show  that  the  low  radar  reflectivity  of  Venus  rules  out  any 
possibility  of  large  bodies  of  water  being  on  the  surface. 

Carpenter  and  Goldstein,  by  radar  observations  of  Venus,  establish  its  rotation  as 
being  retrograde  with  a  period  of  approximately  240  days. 

1964  Deirmendjian  proposes  that  the  clouds  are  composed  of  water. 

1966  Ash,  Shapiro,  and  Smith  analyze  radar  data  and  conclude  that  the  diameter  of 

Venus  is  12,112  km  (7526  mi.). 

1966  Boyer  and  Guerin  determine  a  cloud  circulation  of  about  4  days  from  a  study  of 

ultraviolet  photographs. 

1966  Connes  measures  traces  of  HC1  and  HF  in  the  atmosphere. 

1967  Kuiper  makes  the  first  airborne  observations  of  Venus. 

1968  Eshleman  and  colleagues  estimate  surface  temperature  and  pressure  from  radio, 
radar,  and  Venus  probe  data  as  427°C  (800°F)  and  100  atm. 

1970  Singer  suggests  that  Venus  lost  its  initial  spin  and  obtained  its  present  slow 

retrograde  spin  by  impact  of  a  satellite  in  a  retrograde  orbit. 

1973  Young  and  Sill  propose  that  the  clouds  of  Venus  consist  of  drops  of  sulfuric  acid. 

1973  Pollack  observes  Venus  from  a  high-flying  aircraft  observatory  and  concludes 

that  clouds  are  deep  hazes  of  sulfuric-acid  drops. 

1973  Young  describes  observations  of  carbon-dioxide  absorptions  in  the  Venus 

atmosphere  that  show  a  20%  fluctuation  of  a  4-day  period  which  represents 
upward  and  downward  motions  of  the  cloud  deck  on  a  planetwide  scale. 

1973  Goldstein's  radar  scans  of  Venus  reveal  huge,  shallow  craters  on  its  surface. 

1974  Goldstein  produces  high-resolution  radar  images  of  small  areas  of  the  planet's 
surface  showing  many  topographic  features. 

1976  Carbon  monoxide  is  detected  in  the  upper  atmosphere  of  Venus  by  Kitt  Peak 
National  Observatory.  This  gas  had  been  detected  earlier  at  lower  altitudes 
through  infrared  spectroscopy. 

1977  Radar  images  obtained  at  Arecibo  indicate  large  volcanoes  and  craters  on 
the  planet. 

1978  Barker  identifies  carbonyl  sulfide  in  the  Venus  atmosphere. 

1979  Sulfur  dioxide  is  discovered  in  the  atmosphere  by  observations  from  an  28 1 
ultraviolet  satellite. 




M.  E.  Strobell  and  Harold  Masursky 
U.S.  Geological  Survey 
Flagstaff,  Arizona 

During  the  last  five  years,  committees  of  the 
International  Astronomical  Union  (IAU) 
(1980)  have  chosen  and  approved  names  of 
Venusian  surface  features  that  appear  on 
recently  published  maps  (Masursky  et  al., 
1980;  Pettengill  et  al.,  1980;  U.S.  Geological 
Survey,  1981)  and  a  globe  (U.S.  Geological 
Survey  and  Massachusetts  Institute  of  Technol- 
ogy, 1981).  IAU  committees  developed  this 
nomenclature,  or  naming  system,  to  encour- 
age scientists  to  discuss  Venus'  features.  These 
included  surface  features,  physical,  chemical, 
and  mechanical  surface  processes,  and  condi- 
tions within  the  planet's  interior.  All  these 
features  have  led  to  the  planet's  present 
surface  configuration. 

Clouds  and  a  dense  atmosphere  hide  Venus' 
surface  from  visual  observations.  This  fact 
prevented  scientists  from  developing  a  naming 
system  like  the  other  terrestrial  planets'  before 
the  mid-1960s.  Early  in  the  decade,  monostatic 
and  pulsed  Earth-based  radar  systems  were 
able  to  detect  echoes  from  Venus'  surface. 
With  these  systems,  researchers  were  able  to 
determine  its  spin-axis  orientation  and  period 
of  rotation. 

At  the  same  time,  investigators  recognized 
certain  areas  of  anomalous,  or  unusual, 
reflectivity  and  brightness.  Goldstein  (1965) 
named  the  two  brightest  areas,  which  appear 
in  images  that  California's  Jet  Propulsion 
Laboratory  took  in  1964,  "Alpha"  and  "Beta." 
During  the  mid-  and  late- 1960s,  workers  at 
other  facilities  (Carpenter,  1966;  Dyce, 
Pettengill,  and  Shapiro,  1967;  Rogers  and 

Ingalls,  1969)  confirmed  these  and  other 
anomalous  areas.  At  that  time,  each  radar 
facility  had  its  own  informal  naming  system 
(Carpenter,  1966).  In  1967,  astronomers  at  the 
Arecibo  facility,  Puerto  Rico,  informally  named 
features  with  high-delay  Doppler  frequencies 
for  renowned  physicists.  They  named  one  such 
feature,  which  they  had  not  recognized 
previously,  "Maxwell"  (Jurgens,  1970).  By 
1969,  scientists  had  recognized  circular  areas 
of  very  low  reflectivity  (Rogers,  Ingalls,  and 
Pettengill,  1974).  In  the  early  1970s,  they 
discriminated  other  irregular  and  elongate 
features  on  higher-resolution  images. 

When  NASA  completed  plans  for  the  Pioneer 
Venus  mission,  a  Task  Group  for  Venus 
nomenclature  formed.  It  was  established  under 
the  direction  of  the  Working  Group  for 
Planetary  System  Nomenclature  of  the  IAU. 
This  task  group's  goal  was  to  create  a  system- 
atic plan  for  naming  the  features  clarified  by 
Pioneer  Venus'  altimetric  and  imaging  sys- 
tems. The  group's  goal  also  included  naming 
those  features  appearing  in  a  growing  number 
of  high-resolution  Earth-based  images. 

The  Task  Group  chose  a  theme  in  keeping  with 
Venus'  age-old  feminine  mystique  (Table  B-l, 
compiled  by  L.  Colin).  They  named  features 
for  females,  both  mythological  and  real,  who 
appear  in  the  mythologies  and  histories  of  all 
world  cultures.  Circular,  crater  like  features 
would  be  named  for  notable  historical  women 
while  other  features  would  bear  the  names  of 
goddesses  and  heroines  from  myth  and  legend 
(IAU,  1977).  The  exceptions  were  "Alpha," 


Table  B-1.  Venus  Mythology 


Venus  —  Roman  goddess  of  love  and  beauty,  grace,  fertility 
Vesper  —  Latin,  ancient  Roman,  evening  star 
Lucifer  —  Latin,  ancient  Roman,  morning  star 

Aphrodite  —  Greek  goddess  of  love,  beauty,  fruitfulness 
Hesperos  —  Ancient  Greek,  evening  star 
Phosphoros  —  Ancient  Greek,  morning  star 

Quaiti  —  Egypt,  evening  star 
Tioumoutiri  —  Egypt,  morning  star 

Ruda  —  Arab,  evening  star 

Helel  —  Hebrew,  morning  star 

Ishtar  (Istar)  —  Babylonia,  Assyria,  Mylitta,  Chaldea,  Sumeria 
Astarte  (Ashtarte)  —  Caanan,  Phoenicia,  Aramean,  Southern  Arabs,  Egyptians 
Athtar  (Allat)  —  Arab 
Ashtoreth  —  Biblical  Israelite  pagans 

Anahita  —  Persia 

Five  names  above  are  pagan  Semitic  goddesses  of  love,  fertility,  maternity,  sexual  activity, 
and  war. 

Tai-pe  —  China,  beautiful  white  one 

Freya  (Freyja)  —  Teutonic  goddess  of  love,  beauty,  fertility 
Frija  —  Old  German 

Frig  (Friga)  —  Anglo-Saxon;  Friday  -  6th  day  of  week 
Frigg  (Freia)  —  Old  Norse 

Chasca  —  Inca,  goddess  of  love 
Tlazolteotl  —  Mexico,  goddess  of  love 
Quetzalcoatl-Kukulcan  —  Post-classic  Maya,  lord  of  dawn 

Noh  Ek  (Great  Star),  Chac  Ek  (Red  Star),  Sastal  Ek  (Bright  Star),  Ah  Sahcab  (Companion  of 
the  Aurora),  Xux  Ek  (Wasp  Star)  —  Mayan  Venus 

Cythera  —  Island  birthplace  of  Venus 

"Beta,"  and  "Maxwell."  These  three  names 
remained  because  they  were,  by  then,  firmly 
established  in  radar  literature.  The  Task  Group 
compiled  an  extensive  list  of  female  names 
that  they  could  use.  As  scientists  reduced 
Pioneer  Venus  data  to  map  format,  they 
applied  names  from  the  list  to  obvious  features 
on  those  maps. 

Two  distinctive  features  stand  out  on  Venus' 
topographic,  reflectivity,  and  image  maps. 
They  are  large  radar-bright  areas  of  highland 
terrain  the  size  of  terrestrial  continents.  These 
areas — Ishtar  (Babylonian)  and  Aphrodite 
(Greek)  Terrae — were  named  for  goddesses  of 
love.  Ishtar  Terra  also  is  eye-catching  on  Earth- 
based  images  and  on  a  mosaic  that  the 

astronomers  at  Arecibo  compiled  (Campbell 
and  Burns,  1980). 

Linear  highland  regions,  which  usually  also  are 
radar  bright,  take  their  names  from  other 
goddesses.  Examples  are  Akna  and  Freyja 
Montes  (mountains).  Akna  was  the  goddess  of 
birth  worshipped  in  Yucatan.  Freyja  was  the 
principal  Norse  goddess  and  mother  of  Odin 
(Maxwell  Montes  is  an  exception).  A  high, 
relatively  flat,  and  radar-dark  area  has  the 
name  Lakshmi  Planum  (plateau)  to  honor  the 
Indian  goddess  of  prosperity  and  fortune. 

Low  quasi-circular,  or  elongate,  lowland  plains 
that  are  generally  radar  dark  have  names  from 
mythological  heroines.  For  example,  Helen 
Planitia  (plain)  is  the  name  of  the  lady  whose 
face  "launched  a  thousand  ships,"  and  Sedna 
Planitia  honors  a  beautiful  Eskimo  girl. 
Linear  clefts,  or  canyons  (chasmata),  in  Venus' 
surface  are  named  for  goddesses  of  the  hunt  or 
Moon.  A  single  personage  often  had  both 
attributes:  Artemis  was  the  Greek  goddess  of 
the  hunt  and  of  the  Moon.  Diana  was  her 
Roman  counterpart. 

Radar-bright  linear  features  that  coincide  with 
an  abrupt  topographic  change,  such  as  a  cliff 
(rupes),  are  named  for  hearth  goddesses.  For 
example,  Vesta  Rupes  was  named  for  the 
Roman  goddess. 

All  circular  features  received  names  of  notable 
deceased  women.  Irregular  craters  at  or  near 
mountain  summits  assume  the  names  of 
classical  women.  For  example,  Sappho  Patera 
has  the  name  of  the  Greek  poetess.  Craters  in 
plains  areas  received  their  names  from  modern 
women,  such  as  the  physicist  Lise  Meitner. 

Scientists  have  traditionally  applied  the  term 
"Regio"  to  any  feature  on  a  planetary  surface 
that  they  cannot  clearly  define  or  understand, 

usually  because  image  resolutions  are  not  clear 
enough.  They  applied  the  term  first  to  the 
albedo  features  on  Mars  and,  more  recently,  to 
dark  regions  appearing  on  Voyager  images  of 
Ganymede.  On  Venus,  astronomers  originally 
used  the  term  to  describe  the  radar-bright 
features  Alpha  and  Beta  that  Earth-based  radar 
systems  identified.  The  term  now  includes 
regions  of  somewhat  elevated  terrain  that  are 
smaller  than  continents  but  do  not  necessarily 
appear  as  discrete  features  on  other  data  sets. 
These  features  take  their  names  from  titanesses 
and  giantesses. 

Other  features  (the  radar-bright  linear  regions 
we  know  as  lineae,  or  lines),  have  very  low 
topographic  expression  at  Pioneer  Venus 
resolutions.  Because  of  their  low  resolution, 
they  are  well  known  only  in  reflectivity 
images.  These  features  are  named  for  goddesses 
and  heroines  of  war.  These  include  women 
such  as  Hippolyta,  the  Greek  leader  of  the 
Amazons,  and  Vehansa,  the  Teutonic  war 
goddess.  Features  that  are  now  designated  as  a 
linea  or  regio  (region)  may  receive  other 
generic  feature  designations  at  a  later  date. 
This  can  happen  if  future  radar  missions 
obtain  higher  resolution  data  that  clarify  these 
regions'  true  geomorphic  expression. 

Names  of  Venusian  features  appear  in 
Table  B-2. 


Table  B-2.  Venus  Nomenclature  Assigned 






Chasmata  (goddess  of  the  hunt;  Moon  goddess);  canyons 


21  S 
0-1  2N 


Goddess  of  the  hunt/Moon 
Goddess  of  the  hunt 
Goddess  of  the  hunt 
Goddess  of  the  hunt/Moon 
Chinese  Moon  goddess 

Craters  (modern  notable  women) 




Symbolizes  the  first  biblical  woman 
French  novelist  and  writer 




German-Swedish  physicist  (1  878-1  968) 




Shoshone  Indian  guide  to  the  Lewis  and 
Clark  expedition  to  the  Pacific  Northwest  (1  786-1812) 

Patera  (classical  notable  woman);  irregular,  possibly  volcanic  craters 


65  N 


Famous  Egyptian  queen;  notable  for  her  love  affairs  with 

Julius  Caesar  and  Mark  Anthony  (69  B.C.-30  B.C.) 




Greek  lyric  poetess  of  great  power  (580-610  B.C.) 




Wife  of  Justinian;  most  famous  and  powerful  woman  in 

Byzantine  history.  Influential  in  passing  laws  that  first 

recognized  rights  of  woman  (508-548  B.C.) 

Linea  (goddess  and  heroine  of  war);  lines 








Valkyrie;  Norse  female  warrior;  means  "battle" 




Germanic  war  goddess 








Valkyrie  maiden  who,  in  Icelandic  legend,  sang  so 

sweetly  that  the  enemy  could  not  defend  themselves 








War  goddess 

Monies;  mountains 




Yucatan;  goddess  of  birth 




Mother  of  Odin  in  Teutonic  mythology 




Ancient  Egyptian  goddess  of  the  sky 




James  C.  Maxwell;  British  physicist  (1831-1879) 




Titaness;  Earth  goddess 




Titaness  in  Greek  mythology 

Table  B-2.  Venus  Nomenclature  Assigned  (Cont.) 





Planum;  plateaux 




Indian  goddess  of  fortune  and  prosperity 

Plainitiae  (heroines) 




Finnish  heroine;  Vainamoinen,  one  of  the  Kalevala 

heroes,  wished  to  marry  her;  she  became  a  water 

divinity  and  thus  escaped  him 




Atalanta  swore  she  would  only  marry  the  man  who  could 

beat  her  at  a  footrace.  Melanion  dropped  three  golden 

apples  during  the  race  and  was  able  to  win  the  race 

when  Atalanta  stopped  to  pick  them  up 




Wife  of  King  Arthur  and  beloved  of  Lancelot 




Wife  of  Menelaus;  Paris,  son  of  Priam  of  Troy,  fell  in  love 

with  her  and  carried  her  off  to  Troy,  thus  precipitating 

the  Trojan  War 




Wife  of  Aeneas 




Wife  of  Tyndareus;  Zeus,  enamored  of  her  charms, 

disguised  himself  as  a  beautiful  swan  and  seduced  her. 

She  gave  birth  to  Pollux  and  Helen  (by  Zeus)  and  Castor 

Clytemnestra  (by  Tyndareus) 




Wife  of  Amphion  of  Thebes.  She  gave  birth  to  1  2 

children,  who  were  all  killed  by  Artemus  and  Apollo 




A  beautiful  Eskimo  girl,  who  was  wooed  and  won  by  a 

phantom  bird  who  carried  her  off  to  a  far  shore.  Sedna's 

father  followed  them,  stole  Sedna  back,  and  started 

home  with  her.  The  phantom  bird  made  a  great  storm 

come  up,  and  the  father,  in  fear,  threw  Sedna  into  the 

ocean.  When  she  tried  to  climb  back  in  the  kayak,  her 

father  cut  off  parts  of  her  fingers,  which  became  seals, 

walruses,  and  whales 

Regiones  (alphanumeric;  female  titans);  regions 




First  letter  in  the  Greek  alphabet 


1  8-30N 


Greek  titaness 




Second  letter  in  the  Greek  alphabet 




Norse  giantess 




Norse  giantess 




Greek  titaness 




Greek  titaness 




Greek  titaness 




Greek  titaness 




Greek  titaness 




Norse  giantess 


Rupes  (goddess  of  hearth,  home);  cliffs 



48-5  3  N 
55-65  N 


Turco-Tatar  goddess  of  the  hearth  fire 
Roman  hearth  goddess 

Terrae  (goddess  of  love);  continents 




Greek  goddess  of  love 
Babylonian  goddess  of  love 



Campbell,  D.  B.;  and  Burns,  B.  A.:  Earth-based 
Radar  Imagery  of  Venus.  J.  Geophys.  Res., 
vol.  85,  no.  A13,  1980,  pp.  8271-8281. 

Carpenter,  R.  L.;  and  Department  of  Astron- 
omy, UCLA:  Study  of  Venus  by  CW  Radar— 
1964  Results.  Astron.  J.,  vol.  71  ,  no.  2,  1966, 
pp.  142-152. 

Dyce,  R.  B.;  Pettengill,  G.  H.;  and  Shapiro,  1. 1.: 
Radar  Determination  of  the  Rotations  of  Venus 
and  Mercury.  Astron.  J.,  vol.  72,  no.  3,  1967, 
pp.  351-359. 

Goldstein,  R.  M.:  Preliminary  Venus  Radar 
Results.  J.  Res.,  sec.  D,  Radio  Science,  vol.  69D, 
no.  12,  Dec.  1965,  pp.  1623-1625. 

Jurgens,  R.  F.:  Some  Preliminary  Results  of  the 
70-cm  Radar  Studies  of  Venus.  Radio  Science, 
vol.  5,  no.  2,  1970,  pp.  435-442. 

Masursky,  Harold;  et.  al.:  Pioneer  Venus  Radar 
Results:  Geology  from  Images  and  Altimetry. 
J.  Geophys.  Res.,  vol.  85,  no.  A13,  1980, 
pp. 8232-8260. 

Pettengill,  G.  H.;  et.  al.:  Pioneer  Venus  Radar 
Results:  Altimetry  and  Surface  Properties. 
J.  Geophys.  Res.,  vol.  85,  no.  A13,  1980, 
pp.  8261-8270. 

Rogers,  A.  E.  E.;  and  Ingalls,  R.  P.:  Venus- 
Mapping  the  Surface  Reflectivity  by  Radar 
Interferometry.  Science,  vol.  165,  no.  3895, 
1969,  pp.  797-799. 

Rogers,  A.  E.  E.;  Ingalls,  R.  P.;  and  Pettengill, 
G.  H.:  Radar  Map  of  Venus  at  3.8  cm 
Wavelength.  Icarus,  vol.  21,  no  3,  1974, 
pp.  237-241. 

U.S.  Geological  Survey:  Altimetric  and  Shaded 
Relief  Map  of  Venus:  U.S.  Geological  Survey 
Miscellaneous  Investigations  Series 
Map  1-1324,  1981. 

U.S.  Geological  Survey  and  Massachusetts 
Institute  of  Technology:  Globe  of  Venus. 
Replogie  Globes,  Inc.,  1981. 

Working  Group  for  Planetary  System  Nomen- 
clature. IAU  Trans.,  vol.  16B,  1977, 
pp.  321-369. 

Working  Group  for  Planetary  System  Nomen- 
clature. IAU  Trans.,  vol.  17B,  1980, 
pp.  285-304. 

Additional  Reading 

Aveni,  A.  F.:  Venus  and  the  Maya.  American 
Scientist,  vol.  67,  no.  3,  May-June  1979, 
pp.  274-285. 

Azimov,  I.:  Venus,  Near  Neighbor  of  the  Sun. 
Lothrop,  Lee  &  Shepard,  1981. 

Durant,  W.:  Our  Oriental  Heritage:  Being  a 
History  of  Civilization  in  Egypt  and  the  Near 
East  to  the  Death  of  Alexander,  and  in  India, 
China  and  Japan  from  the  Beginning  to  Our 
Own  Day.  Simon  and  Schuster,  1954. 

Hawkes,  J.:  The  Atlas  of  Early  Man.  St.  Martin's 
Press,  1976. 

Moore,  P.:  The  Planet  Venus.  Second  ed. 
Macmillan,  1959. 

Wilson,  C.:  The  Starseekers.  First  ed. 
Doubleday,  1980. 



A.  Rules  of  the  Road  for  Pioneer 
Venus  Investigators 

The  Pioneer  Venus  Science  Steering  Group 
(PVSSG)  developed  a  set  of  procedures  and 
rules  to  assure  an  orderly  and  efficient  analysis 
and  interpretation  of  the  mission's  scientific 
results.  These  rules  appear  here  for  historical 
interest  and  for  possible  use  in  future  projects. 

1.  Instrument  Principal  Investigators,  Radio 
and  Radar  Science  Team  members,  and  Inter- 
disciplinary Scientists  (including  the  project 
scientist  for  the  purpose  of  these  rules)  were 
designated  Pioneer  Venus  Investigators  (PVIs). 
Only  PVIs  could  sponsor  investigators 
(research  projects  involving  unpublished 
Pioneer  Venus  data).  (Later,  the  Principal 
Investigators  were  Pis,  Radar  and  Radio 
Scientists  were  Radioscience  Investigators  (RIs), 
and  Interdisciplinary  Scientists  were  IDS. 

A  planned  Guest  Investigator  (GI)  program 
began  in  1981.) 

2.  Each  instrument  PVI  was  responsible  for 
analysis  and  interpretation  of  data  from  his 
instrument.  He  and  his  co-investigators  (COIs, 
originally  Co-Is)  were  responsible  for  the 
initial  analysis,  interpretation,  and  publication 
of  those  data.  The  three  months  following  the 
acquisition  of  any  data  by  the  PVI  were 
important.  During  that  time,  he  identified  the 
investigation  that  he,  his  COIs,  and  associates 
expected  to  pursue  with  those  data.  (Associates 
were  people  such  as  graduate  students  or  post- 
doctoral research  fellows  who  were  clearly 
identified  with  the  PVI  or  his  COIs.  Normally, 
the  criterion  was  funding  for  their  salaries 
through  PV  data  analysis  contracts.  They  were 
specifically  not  senior,  independent  scientists 
who  belonged  to  the  same  institution  as  the 
PVI  or  COI.) 

3.  PVIs  and  COIs  had  free  access  to  all  data 
acquired  during  the  mission  (and  extended 
mission).  They  also  had  access  to  publications 
resulting  from  the  use  of  those  data.  The 
normal  vehicle  for  data  dissemination  was  the 
Unified  Abstract  Data  System. 

4.  If  a  PVI's  unpublished  data  were  used  in  an 
investigation,  that  PVI  had  the  right  to  be 
included  among  the  authors  of  any  publica- 
tion that  resulted.  During  the  formative  stages 
of  an  investigation,  it  was  the  sponsoring 
investigator's  responsibility  to  solicit  the  par- 
ticipation of  the  PVI  whose  data  or  results  he 
was  using.  The  PVI,  whose  cooperation  the 
investigator  solicited,  could  refuse  coauthor- 
ship  but  not  use  of  his  data.  He  had,  however, 
to  provide  information  concerning  the  quality 
of  the  data  in  question.  He  also  could  require 
that  the  sponsoring  investigator  include 
suitable  caveats  regarding  the  data 

in  publications. 

5.  The  role  of  an  Interdisciplinary  Scientist 
(IDS)  in  this  mission  was  to  enhance  the 
scientific  output  of  the  mission.  He  did  this  by 
promoting  investigations  that  used  data  from 
more  than  one  instrument.  Mission  personnel 
hoped  that  the  IDS  would  be  able  to  promote 
cooperation  among  other  Pis.  They  also  hoped 
that  other  PVIs  would  exploit  any  unusual 
insights  the  IDS  had.  In  this  way,  they  would 
enrich  interpretation  of  data  from  specific 
instruments  as  well  as  from  an  ensemble  of 
instruments.  Thus,  administration  normally 
expected  IDSs  to  participate  in  investigations 
that  involved  data  from  more  than  one 
instrument.  This  could  occur  either  as  a  result 
of  their  proposing  such  investigations  or  by 
invitation  from  other  PVIs  to  participate. 



Suppose  a  group  of  PVIs  proposed  an  investiga- 
tion in  an  area  in  which  an  IDS  was  a  special- 
ist. Normal  procedure  was  to  invite  him  to 
participate.  After  the  three-month  period  in 
Rule  2,  an  IDS  could  propose  an  investigation 
involving  data  that  a  single  instrument 
produced.  COIs  of  the  PVI  responsible  for  that 
instrument  also  had  a  right  to  participate  in 
that  investigation.  They  could  ask  their 
associates  to  participate,  too. 

6.  PVIs  or  COIs  could  not  preempt  major 
science  areas  for  themselves.  They  had  to 
pursue  an  investigation  promptly. 

7.  Scientific  Working  Groups  normally  pro- 
vided the  forum  in  which  researchers  discussed 
investigations.  Researchers  had  to  send  titles 
and  descriptions  of  proposed  investigations  to 
the  Project  Scientist.  He  served  as  the  interface 
between  investigators,  project,  and  other  PVIs. 
In  particular,  he  informed  all  PVIs  of  proposed 
new  investigations.  Objections  or  comments 
by  other  PVIs  went  to  the  Science  Steering 
Group's  co-chairmen  for  settlement  or  other 
appropriate  action. 

8.  PVIs  could  release  their  own  data  to  whom- 
ever they  wished,  but  not  the  data  of  other 
PVIs  without  their  consent. 

9.  There  was  no  Pioneer  Venus  mission  policy 
for  paper  form  or  publication  medium.  The 
only  exception  was  a  possible  agreement  for 
publication  of  the  mission's  initial  results. 

10.  Independent  scientists  who  were  not 
mission  PVIs,  COIs,  or  associates  could 
participate  in  an  investigation  provided: 

(a)  A  PVI  sponsored  them. 

(b)  They  provided  suitable  correlative  data 
for  distribution  to  other  PVIs  through  the 
sponsoring  PVI. 

(c)  The  rest  of  the  PVIs  gave  their  approval 
before  the  investigation  started,  and  the  SSG 
issued  a  letter  of  invitation  and  cooperation. 
Such  scientists  later  formed  the  program's  GIs. 
This  group  began  in  1981.  The  objective  was  to 
involve  new  scientists  in  the  program  who 
would  bring  a  fresh  perspective  to  data 
analysis  and  interpretation. 

B.  Pioneer  Venus  Working  Groups 

The  PVSSG  developed  a  set  of  six  Working 
Groups  to  address  particular  disciplines.  These 
disciplines  were  Composition  and  Atmosphere 
Structure;  Clouds;  Dynamics;  Thermal  Balance; 
Solar  Wind,  Ionosphere,  and  Aeronomy;  and, 
Surface  and  Interior.  The  Working  Groups 
were  very  successful  and  wrote  group  papers 
synthesizing  results  from  various  experiments. 

Composition/ Atmosphere  Structure 


J.  Hoffman  (LNMS)— Chairman 

A.  Stewart  (OUVS) 

V.  Oyama  (LGC) 

U.  von  Zahn  (BNMS) 

H.  Niemann  (ONMS) 

A.  Seiff  (LAS/SAS) 

D.  Hunten  (IS) 

N.  Spencer  (IS) 

T.  Donahue  (IS) 

G.  Keating  (RADIO) 

A.  Kliore  (RADIO) 


F.  Taylor  (OIR) 

R.  Knollenberg  (LCPS) 

H.  Taylor  (OIMS) 

R.  Goody  (IS) 

A.  Nagy  (IS) 

J.  Pollack  (IS) 

T.  Croft  (RADIO) 



R.  Knollenberg  (LCPS)— Chairman 

R.  Ragent  (LN/SN) 

F.  Taylor  (OIR) 

J.  Hansen  (OCPP) 

A.  Stewart  (OUVS) 
V.  Oyama  (LGC) 
M.  Tomasko  (LSFR) 
V.  Suomi  (SNFR) 
D.  Hunten  (IS) 
J.  Pollack  (IS) 
T.  Croft  (RADIO) 



G.  Schubert  (IS)— Chairman 
C.  Counselman  (DLBI) 

F.  Taylor  (OIR) 
A.  Seiff  (LAS/SAS) 
J.  Hansen  (OCPP) 
R.  Woo  (RADIO) 
T.  Croft  (RADIO) 


G.  Pettengill  (RADIO) 
U.  von  Zahn  (BNMS) 
V.  Suomi  (SNFR) 

R.  Goody  (IS) 
J.  Pollack  (IS) 

Thermal  Balance 


M.  Tomasko  (LSFR) — Chairman 

F.  Taylor  (OIR) 

R.  Boese  (LIR) 

A.  Seiff  (LAS/SAS) 

J.  Hansen  (OCPP) 

V.  Suomi  (SNFR) 

R.  Goody  (IS) 
J.  Pollack  (IS) 

A.  Stewart  (OUVS) 
V.  Oyama  (LGC) 
D.  Hunten  (IS) 

Solar  Wind/Ionosphere  Aeronomy 


S.  Bauer  (IS)  (later  A.  Nagy  (IS)— Chairman 

I.  Stewart  (OUVS) 

F.  Scarf  (OEPD) 

C.  Russell  (OMAG) 
L.  Brace  (OETP) 

H.  Taylor  (OIMS) 

W.  Knudsen  (ORPA) 

A.  Barnes  (OP A)  formerly  J.  Wolfe  (OPA) 

N.  Spencer  (IS) 

T.  Donahue  (IS) 

T.  Croft  (RADIO) 


U.  von  Zahn  (BNMS) 

H.  Niemann  (ONMS) 

D.  Hunten  (IS) 

G.  Keating  (RADIO) 
A.  Kliore  (RADIO) 



H.  Masursky  (IS) — Chairman 

C.  Russell  (OMAG) 

G.  Pettengill  (ORAD) 

W.  Kaula  (ORAD) 

G.  McGill  (IS) 

R.  Phillips  (RADIO) 

I.  Shapiro  (RADIO) 


V.  Oyama  (LGC) 

G.  Schubert  (IS) 


C.  Key  Scientific  Questions 

Prior  to  the  Pioneer  spacecraft's  launch,  the  six 
PVSSG  Working  Groups  each  developed  a  set 
of  key  scientific  questions.  These  were  ques- 
tions that  their  members  and  the  associated 
experiments  could  and  would  address  during 
the  mission. 

Composition/Atmosphere  Structure 

Key  Questions 

•  Present  state  of  atmosphere 

Lower  atmosphere  composition 

-  Apart  from  CO2,  what  does  the  lower 
atmosphere  consist  of,  and  how  are  these 
constituents  distributed? 

-  What  are  the  clouds  made  of? 

-  What  does  the  atmosphere  tell  us  about 
the  planet's  surface  and  interior? 

Lower  atmosphere  structure 

-  How  do  the  state  property  profiles  vary 
over  the  planet? 

-  Why  is  the  lower  atmosphere  so  hot? 

-  What  role  do  phase  changes  play  in  the 
thermal  structure? 

Upper  atmosphere  composition  and  structure 

-  What  are  the  composition  and  temperature 
profiles  of  the  upper  atmosphere  and  where 
is  the  homopause? 

-  What  are  the  spatial  and  temporal  varia- 
tions in  Venus'  upper  atmosphere? 

-  Is  the  stability  of  CO2  due  to  global 
circulation  or  local  turbulence? 

-  How  does  the  neutral  composition 
influence  the  ionosphere  and  the 
thermal  structure? 

-  Does  superrotation  extend  into  the 

-  How  does  the  upper  atmosphere  respond 
to  changes  in  solar  extreme  ultraviolet 
radiation  and  solar  wind? 

•  Origin  and  evolution  of  Venus'  atmosphere 

-  Where  did  the  atmosphere  come  from  and 
where  is  it  going? 

-  Where  is  the  water? 

-  Why  does  Venus'  atmosphere  differ  so 
much  from  Earth's? 


Key  Questions 

•  What  is  the  planetary  cloud  structure  in 
altitude  and  horizontally? 

•  How  deep  do  the  sulfuric  acid  clouds  extend? 

•  Do  larger  particles  or  denser  clouds  (higher 
concentration)  exist  at  lower  levels?  What  is 
their  composition? 

•  Is  the  concentration  of  cloud  particles 
proportional  to  gas  pressure  so  that  the  scale 
heights  of  the  particles  and  gas  are  identical? 

•  What  substance  is  responsible  for  the 
ultraviolet  absorption  contrasts?  Is  the 
ultraviolet  absorber  well-mixed  vertically 
and  not  horizontally? 

•  What  is  the  structure  and  composition  of  the 
thin  haze  layers  above  the  visible  cloud  deck 
(70-90  km  (43-56  miles))?  Do  they  correlate 
with  the  Mariner  10  radio-occultation 

•  What  is  the  nature  of  the  observed  white 
polar  caps? 

•  Is  there  aeolian  transport  of  dust  within 
10  km  (6  miles)  of  the  surface? 

•  What  are  the  couplings  between  cloud 
microphysics  and  Venusian  dynamics? 

•  What  are  the  cloud  optical  properties? 

•  What  are  the  cloud  formation  and  dissipation 
mechanisms?  Coalescence?  Coagulation? 
Condensation?  Evaporation?  Precipitation? 

•  Why  is  the  cloud  size  spectrum  so  narrow? 


Key  Questions 

•  Upper  atmosphere  circulation 

Is  the  apparent  four-day  rotation  an  actual 
zonal  motion  of  the  atmosphere  or  is  it  a 
wave  phenomenon? 

Do  retrograde  100  m/sec  upper  atmosphere 
zonal  winds  flow  all  around  the  planet, 
even  in  the  antisolar  region? 

Is  there  a  longitude-dependence  of  the 
zonal  motion  speed,  especially  with  respect 
to  the  subsolar  region? 

What  is  the  latitude-dependence  of  the 
apparent  zonal  wind  velocities? 

What  is  the  altitude-dependence  of  zonal 
wind  velocities?  Is  there  essentially  a 
decoupling  of  the  upper  atmosphere  from 
the  lower,  with  large  zonal  winds  confined 
mainly  to  the  upper  atmosphere? 

What  are  the  magnitudes  of  meridional 

What  mechanism  drives  the  rapid  zonal 
circulation  of  the  upper  atmosphere? 

•  Lower  atmosphere  circulation 

What  is  the  nature  of  the  lower  atmosphere's 

Are  the  motions  primarily  zonal  or 

What  is  the  magnitude  of  the  velocity? 

If  the  motions  are  meridional,  do  they 
represent  a  Hadley  cell  circulation? 

If  the  motions  are  zonal,  is  there  an  overall 
rotation  of  the  lower  atmosphere?  Or  is 
the  circulation  between  subsolar  and 
antisolar  points? 

Are  there  unique  motions  (for  example, 
small-scale  convection)  near  the  subsolar, 
antisolar,  and  polar  regions  in  the  deep 

•  Vertical  flow  and  convection 

Are  there  strong  upward  and  downward 
convective  motions? 

What  are  the  horizontal  scales  of 
convective  cells? 

What  are  the  magnitudes  of  vertical 

•  Waves  and  instabilities 

Are  there  any  wave-like  phenomena  or 
instabilities  that  scientists  can  identify  as 
occurring  in  the  atmosphere? 

•  Distinctive  features  in  the  Mariner  10 

What  atmospheric  processes  are  responsible 
for  circumequatorial  belts,  bow  waves,  spiral 
streaks,  polar  ring,  and  other  distinctive 
features  in  Mariner  10  pictures? 


1  Turbulence  and  eddy  diffusion 

Why  is  the  exospheric  temperature  so  low? 


What  is  the  intensity  of  turbulence  in  the 
atmosphere?  What  are  the  altitudes  of  turbu- 
lent layers?  What  are  their  thicknesses?  What 
are  the  turbulent  eddy  diffusion  coefficients? 

•  Thermal  contrast  and  energy  deposition 

What  are  horizontal  temperature  contrasts 
that  drive  atmospheric  motions?  What  is 
distribution  of  solar  energy  deposition  in 
the  atmosphere? 

•  Phase  changes 

Do  phase  changes  and  associated  latent  heats 
of  condensible  species  play  an  important  role 
in  the  atmospheric  dynamics? 

•  Nature  of  Ultraviolet  Clouds 

What  materials  and  physical  processes 
are  responsible  for  the  ultraviolet  albedo 

Thermal  Balance 

Key  Questions 

•  What  is  the  cause  of  the  high  surface  tem- 
perature? If  it  is  a  greenhouse  effect,  what 
are  the  sources,  other  than  CC>2,  of  the 
infrared  opacity? 

•  Why  are  there  small  horizontal  temperature 
contrasts  near  the  cloud  tops  in  the  presence 
of  strong  apparent  motions? 

•  Why  are  there  small  horizontal  temperature 
gradients  (both  day-night  and  equator-pole) 
at  the  cloud  tops  and  near  the  surface? 
(These  occur  despite  an  expected  strong 
variation  in  the  local  deposition  of  solar 
energy  over  the  illuminated  hemisphere.) 

•  What  are  the  roles  of  radiative  and  dynami- 
cal processes  in  maintaining  the  thermal 
balance  of  the  atmosphere? 

•  What  is  the  global  (vertical  and  horizontal) 
temperature  structure?  How  does  dynamical 
heat  transport  determine  it? 

•  Where  are  the  sources  and  sinks  of  heating 
by  solar  and  thermal  radiation  fields? 

•  What  are  the  cloud  optical  properties? 

•  Do  latent  heat  effects  on  convection 
produce  subadiabatic  regions  in  the 
generally  adiabatic-looking  vertical  tem- 
perature profiles?  Is  the  nearly  adiabatic 
structure  due  to  small-scale  convection  or 
planet-wide  circulation? 

Solar  Wind/Ionosphere  Aeronomy 

Key  Questions 

•  Venus  ionosphere 

What  is  the  ion  composition,  and  what 
controls  the  plasma  distribution  of  Venus' 

What  is  the  plasma  temperature  of  Venus' 
ionosphere  and  what  controls  its  thermal 

What  are  the  mechanisms  and  the  signifi- 
cance of  mass,  momentum,  and  energy 
transfer  from  solar  wind  to  the  upper 

•  Solar  wind- Venus  interaction 

Is  there  an  intrinsic  magnetic  field? 

How  do  ionospheric  currents  contribute  to 
the  deflection  of  solar  wind? 

How  important  are  processes  such  as  charge- 
exchange  and  mass-addition? 

What  is  the  source  of  the  dayside 
ionosphere's  variability? 

How  much  solar  wind  does  the  ionosphere 

Is  there  a  magnetotail? 

Is  there  a  plasma  sheet? 

Are  there  substorms  on  Venus? 

How  does  the  plasma  close  behind 
the  planet? 

What  maintains  the  nightside  ionosphere? 

What  produces  the  two  peaks  in  the  electron 
density  profile  in  the  nightside  ionosphere? 
What  causes  their  variability? 

What  is  the  source  of  night-time  airglow  and 
the  ashen  light? 

Is  there  a  boundary  layer  or  rarefaction 
region  in  the  flow? 

How  does  the  Venus  bow  shock  and 
upstream  region  differ  from  Earth's? 


Key  Questions 

•  What  is  the  extent  of  endogenic  activity 

leading  to  tectonics,  crustal  differentiation, 

and  volcanism? 

•  What  is  the  extent  of  exogenic  processes 
such  as  impact  cratering,  weathering,  and 
transportation  and  erosion  of  surface 
materials  by  winds  and  crustal  recycling? 

•  What  is  Venus'  gravity-field  distribution?  Is 
there  evidence  of  density  contrasts? 

•  Are  tectonic  features  evident  on  the  surface: 
arcuate  mountain  systems,  strip-like  faults  of 
large  displacement,  rifts,  volcanic  craters,  or 
chains  of  volcanic  craters? 

•  Does  Venus'  interior  consist  of  an  iron  core 
and  a  mantle  of  magnesium  and  iron  silicates 
(like  Earth)? 

•  What  is,  and  what  is  the  cause  of,  the 
offset  of  the  center  of  mass  from  the 
center  of  figure? 

•  What  is  the  subsurface  temperature  gradient? 
What  has  been  Venus'  thermal  history? 

•  Can  an  exogenic  effect  (such  as  solar  tidal 
torque  or  a  planetesimal  impact)  explain 
Venus'  slow  retrograde  spin? 

•  Does  Venus  possess  an  intrinsic  magnetic 
field?  How  large  is  it? 

•  Is  the  surface  in  thermal  and  chemical 
equilibrium  with  the  lower  atmosphere? 

•  Is  there  a  resonant  lock  between  Venus'  spin 
period  and  the  relative  orbital  motions  of 
Earth  and  Venus? 

•  Is  Venus  further  along  than  Earth  on  the 
evolutionary  path  toward  the  end  of  com- 
plete compositional  stratification  and 
thermal  quiescence? 



G  LO  S  S  ARY 


ablative  material:  material  that  absorbs  heat  by  converting  state  (i.e.,  solid  to  gas)  and  thereby 
carries  the  heat  away. 

adiabatic:  without  a  loss  or  gain  of  heat  from  the  surroundings. 

aeronomy:  study  of  the  upper  regions  of  the  atmosphere  where  there  is  ionization,  dissociation, 
and  chemical  reaction. 

aeroshell:  an  insulating  shell  to  protect  a  spacecraft  from  atmospheric  heating  during  entry  into 
an  atmosphere. 

airglow:  a  quasi-steady  radiation  from  an  atmosphere  arising  from  collisions  among  molecules 
and  atoms  and  distinct  from  aurora. 

albedo:  ratio  of  the  amount  of  electromagnetic  radiation  reflected  by  a  body  to  the  amount 
incident  upon  it.  For  example,  the  albedo  of  Earth  is  34%. 

angstrom:  unit  of  wavelength  of  light  equal  to  10'8  cm  or  3.937  x  10~9  in. 
anisotropic:  exhibiting  different  properties  when  tested  along  different  axes. 
apoapsis:  point  in  an  elliptical  orbit  that  is  most  distant  from  the  center  of  attraction. 
apsides:  the  two  points  in  an  orbit  nearest  and  farthest  from  the  center  of  attraction. 
arcuate  feature:  a  geological  feature  of  bow  shape. 

ashen  light:  a  glow  from  the  dark  side  of  Venus  that  some  scientists  claim  is  observable 
from  Earth. 

astronomical  unit:  the  mean  distance  of  Earth  from  the  Sun,  approximately  149,599,000  km 
(93,000,000  miles). 

Atmosphere  Explorer:  an  Earth-orbiting  satellite  in  the  NASA  series  used  to  explore  the  upper 

ballistic  trajectory:  trajectory  followed  by  a  body  moving  solely  under  the  influence  of  gravity. 
bar:  unit  of  pressure;  104  dyne/cm2. 
basalt:  an  igneous  rock. 
biftlar:  consisting  of  two  wires. 





boundary  layer:  layer  of  fluid  in  immediate  vicinity  of  a  bounding  surface. 

bow  shock:  shock  wave  in  front  of  a  body  at  which  the  velocity  changes  abruptly. 

caldera:  a  roughly  circular  volcanic  depression  whose  diameter  is  many  times  that  of  the  volcanic 

Carboniferous  era:  the  fifth  period  of  the  Paleozoic  Era. 

Cassegrain  telescope:  a  telescope  using  a  primary  and  secondary  mirror. 

cold  trap:  a  location  in  an  atmosphere  where  gases  can  be  trapped  and  prevented  from  rising 
higher  in  that  atmosphere. 

convective  plume:  a  plume  of  hot  magma  rising  from  the  interior  of  a  planet  toward  its  surface. 
corona:  the  outer  visible  envelope  of  the  Sun. 
cryosphere:  cold,  upper  atmospheric  region  of  a  planet. 

deuterium:  heavy  isotope  of  hydrogen  whose  nucleus  contains  a  neutron  in  addition  to  a  proton. 

differentiation:  process  in  which  light  materials  rise  above  heavier  materials  in  a  gravitational 

Doppler  shift:  apparent  change  in  frequency  of  a  vibration  such  as  sound  or  light  or  radio  waves, 
resulting  from  relative  movement  between  the  observer  and  the  source. 

dropsonde:  a  capsule  dropped  from  a  spacecraft  to  investigate  the  atmosphere  of  a  planet. 

dynamo:  a  direct  current  generator  that  converts  mechanical  energy  into  electrical  energy  by 
motion  of  a  conductor  through  magnetic  field  lines. 


ecliptic  plane:  plane  of  Earth's  orbit  around  the  Sun.  Ecliptic  is  the  projection  of  this  plane  on  the 
star  sphere. 

electron:  subatomic  particle  that  possesses  the  smallest  possible  negative  electric  charge. 
electron  density:  a  measure  of  the  number  of  electrons  per  unit  volume  in  an  ionized  gas. 
eV:  electron-volt;  energy  of  an  electron  accelerated  through  a  potential  of  one  volt. 



exosphere:  outermost  regions  of  an  atmosphere  where  the  molecules  and  atoms  travel  in  ballistic 
paths  and  rarely  collide  with  each  other. 

extreme  ultraviolet  radiation:  ultraviolet  radiation  of  very  short  wavelength. 

Faraday  cup:  a  device  to  measure  plasma  properties  over  a  wide  angular  viewpoint. 

flux  rope:  a  unique  magnetic  structure  consisting  of  a  long,  narrow  region  of  strong  magnetic  field 
with  field  lines  twisted  like  the  threads  of  a  rope. 

gamma:  a  measurement  of  magnetic  field  intensity;  10'5  gauss. 

gamma  ray:  electromagnetic  radiation  of  very  short  wavelength  beyond  that  of  x-rays. 

gamma  burst:  intense,  short-lived  pulse  of  gamma  rays  from  deep  space. 

gravity  wave:  a  wave  disturbance  in  a  fluid  in  which  buoyancy,  or  reduced  gravity,  acts  to  restore 
displaced  parts  of  the  fluid  back  to  hydrostatic  equilibrium. 

greenhouse  effect:  condition  in  which  an  atmosphere  can  absorb  more  radiation  than  it  can  emit 
back  into  space,  thus  causing  a  rise  in  temperature  of  that  atmosphere. 

Hadley  cell:  an  atmospheric  circulation  pattern  in  which  heated  atmosphere  rises  at  the  equatorial 
region,  travels  at  a  high  altitude  toward  the  pole  where  it  cools  and  descends  and  then  travels 
back  at  a  low  altitude  to  the  equatorial  region. 

heavy  hydrogen:  deuterium,  an  isotope  of  hydrogen  whose  nucleus  contains  a  neutron  in  addition 
to  a  proton. 

heliocentric:  centered  on  the  Sun. 

high-gain  antenna:  an  antenna  that  is  designed  to  concentrate  electromagnetic  radiation  into  a 

tight  beam. 

hydrogen  coma:  a  region  around  the  head  of  a  comet  that  hydrogen  atoms  occupy. 

hydroxyl:  a  monovalent  chemical  group  consisting  of  a  hydrogen  atom  linked  to  an  oxygen  atom. 

inertial  space:  a  stationary  frame  of  reference;  a  set  of  coordinates  used  for  calculating  trajectories. 





inferior  conjunction:  position  of  a  planet  moving  in  an  orbit  within  that  of  Earth  when  the  planet 
is  aligned  between  Earth  and  Sun. 

ion:  an  atom  or  molecule  that  is  positively  or  negatively  charged. 

ionopause:  boundary  between  the  shocked  solar  wind  and  the  ionosphere  of  a  planet. 

ionosphere  hole:  a  region  of  the  ionosphere  where  the  number  of  ions  is  severely  depleted. 

ionosphere:  region  of  high  atmosphere  in  which  many  of  the  molecules  and  atoms  are  ionized. 

ionotail:  an  ionized  region  extending  on  the  side  of  the  planet  away  from  the  Sun. 

isostatic:  hydrostatic  equilibrium  maintained  by  flow  of  material  from  one  part  to  another. 

isothermal:  thermodynamic  change  of  state  of  a  system  that  takes  place  at  constant  temperature. 

isotope:  atoms  with  the  same  chemical  properties  but  with  different  atomic  weights  because  of  a 
different  number  of  neutrons  in  the  nucleus. 

Langmuir  probe:  a  device  consisting  of  conductors  inserted  in  a  plasma  to  measure  the 
plasma  current. 

line  of  apsides:  the  line  connecting  the  closest  and  most  distant  points  in  an  elliptical  orbit  from 
the  center  of  attraction. 

lithosphere:  outer  rocky  shell  of  a  planetary  body. 

Lyman  alpha:  radiation  with  a  wavelength  of  1216  angstroms  in  the  extreme  ultraviolet  region  of 
the  spectrum;  emitted  by  hydrogen  atoms. 

magma:  hot  volcanic  rock. 

magnetosphere:  the  volume  around  a  planet  affected  by  the  planet's  magnetic  field. 
magnetotail:  magnetic  field  lines  extending  downstream  from  a  planet  away  from  the  Sun. 
mantle:  the  shell  of  a  planetary  body  underlying  the  crust  of  that  body. 

tnesosphere:  atmospheric  shell  in  which  the  temperature  generally  decreases  with 
increasing  height. 



neutral  atmosphere:  atmosphere  that  is  not  ionized. 

oblate:  distorted  from  a  sphere;  equatorial  diameter  exceeds  polar  diameter. 

occultation:  hiding  of  one  celestial  body  by  another  passing  between  that  body  and  the  observer. 

orbital  decay:  loss  of  kinetic  energy  by  an  orbiting  body  so  that  it  moves  inward  toward  the 
center  of  attraction. 

ozone  hole:  a  region  of  the  ozone  layer  in  which  the  amount  of  ozone  is  depleted. 

periapsis:  point  in  an  elliptical  orbit  that  is  closest  to  the  center  of  attraction. 
perihelion  passage:  usually  refers  to  the  closest  point  to  the  Sun  in  an  orbit  of  a  comet. 
perturbation:  disturbance  of  the  orbit  of  one  body  orbiting  another  by  the  gravity  of  a  third  body. 
photodissociation:  breaking  up  of  a  molecule  by  radiation  by  the  absorption  of  a  photon. 
photoelectrons:  electrons  released  when  a  high-energy  photon  hits  an  atom. 
planetary  dynamo:  circulation  within  a  planet  that  produces  a  magnetic  field. 

plasma:  an  electrically  conductive  gas  consisting  of  neutral  particles,  ionized  particles, 
and  free  electrons. 

plasma  wave:  a  wave  motion  within  a  plasma. 

plate  tectonics:  molding  of  a  planetary  surface  by  movement  of  plates  of  crust  powered  by  forces 
acting  from  within  the  planet. 

precession:  change  in  the  direction  of  the  axis  of  a  spinning  body  or  the  alignment  of  an  orbit 
when  acted  upon  by  a  torque. 

prograde:  motion  in  the  usual  direction  of  the  bodies  in  a  given  system. 

radio  occultation:  occultation  of  a  radio  source  by  a  planetary  body. 
radionuclides:  atoms  that  emit  corpuscular  or  electromagnetic  radiation. 




redox  reaction:  chemical  oxidation-reduction  reaction. 

regolith:  surface  material  of  a  planet. 

retrograde:  motion  opposite  to  the  usual  direction  of  the  bodies  in  a  given  system. 

runaway  greenhouse:  a  condition  in  which  the  greenhouse  effect  continues  to  an  extreme;  for 
example,  until  all  oceans  boil  and  water  is  lost  from  a  planet. 

S-band:  radio  frequency  band  about  2.2  GHz  allocated  to  space  communications. 

scale  height:  a  measure  of  the  relationship  between  density  and  temperature  in  an  atmosphere. 

sidereal  day:  a  planet's  period  of  rotation  with  respect  to  the  stars. 

solar  cycle:  the  cycle  of  approximately  1 1  years  over  which  solar  activity  varies  in  a 
repetitive  fashion. 

solar  flare:  a  sudden  outpouring  of  energy  from  the  Sun. 

solar  panel:  panel  on  a  spacecraft  that  converts  solar  radiation  into  electrical  energy. 

solar  wind:  the  blizzard  of  electrons  and  protons  flowing  from  the  Sun  out  across  the 
Solar  System. 

spin  axis:  the  axis  on  which  a  spacecraft  spins  to  stabilize  its  orientation  in  space. 

spreading  center:  location  on  a  planet's  surface  from  which  crustal  material  emerges  from  within 
the  planet  to  spread  on  its  surface. 

sub-spacecraft  point:  the  point  on  the  surface  of  a  planet  immediately  beneath  a  spacecraft. 
subsolar  ionopause:  the  location  in  the  ionopause  immediately  beneath  the  Sun. 


sunspot  number:  a  measure  of  sunspot  activity  based  on  the  numbers  of  individual  spots  and 
groups  of  spots. 

superior  conjunction:  conjunction  of  a  planet  and  the  Sun  when  the  planet  is  on  the  far  side 
of  the  Sun. 

supernova:  intense  disruption  of  a  star  undergoing  gravitational  collapse  with  an  enormous 
explosive  production  of  energy  and  ejection  into  space  of  most  of  the  star's  mass. 

superrotation:  rotation  of  a  planetary  atmosphere  faster  than  the  rotation  of  the  planet. 
synodic  period:  period  of  rotation  with  respect  to  the  Sun. 

tectonics:  molding  of  a  planet's  surface  by  forces  arising  from  its  interior. 

telemetery:  measurement  at  a  distance. 

terminator:  the  boundary  on  a  planet  between  the  sunlit  and  the  dark  hemispheres. 

thermosphere:  hot  region  of  a  planet's  high  atmosphere. 

transfer  ellipse:  an  ellipse  connecting  two  planetary  orbits. 

transit:  passage  of  one  celestial  body  across  the  face  of  another  as  viewed  from  a  third. 

transponder:  a  combined  receiver  and  transmitter  designed  to  transmit  signals  automatically 
when  interrogated. 

troposphere:  region  of  a  planet's  atmosphere  where  weather  occurs. 

ultraviolet:  region  of  radiant  energy  beyond  the  visible  region  of  the  spectrum  with  wavelengths 
between  1000  and  3800  angstroms. 



X-band:  radio-frequency  band  allocated  to  space  radio  communications;  about  8.5  GHz.  y£ 





Over  500  journal  articles,  conference  meeting  papers,  and  reports  on  the  Pioneer  Venus  Program 
have  been  published.  A  complete  bibliography  of  these  publications  is  available  in  two  formats: 

(1)  a  Macintosh  disk  "Pioneering  Venus  Exploration:  A  Bibliography";  and 

(2)  a  printed  document,  NASA/ Ames  Research  Center  Pioneer  Venus  project  document. 
Contact  the  Pioneer  Venus  Mission's  Science  Chief  Dr.  Lawrence  E.  Lasher  at  Ames  Research  Center 
for  instructions  on  how  to  obtain  a  copy  of  the  disk  or  document. 

The  following  bibliography  contains  major  publications  appropriate  to  the  objectives  of  this  NASA 
Special  Publication.  The  references  included  provide  suggestions  for  further  reading  that,  except  for 
the  first  three,  should  be  readily  available.  The  references  are  arranged  chronologically  in  three  groups: 
historical  background;  the  Pioneer  Venus  mission  and  its  results;  and  background  on  Pioneer  Venus 
and  other  Venus  missions. 

Pioneer  Venus  Program:  Historical 

1970       Venus,  Strategy  for  Exploration:  Report  of  a  Study  by  the  Space  Science  Board.  National 
Academy  of  Sciences,  June  1970. 

1972  Pioneer  Venus:  Report  of  a  Study  by  the  Science  Steering  Group,  NASA  TM-X-6237,  June  1972. 

1973  Schuyer,  M.:  Pioneer- Venus  Orbiter:  Assessment  Report  of  Feasibility  Studies  by  MBB,  BAG, 
CIA.  European  Space  Agency,  1973. 

Pioneer  Venus  Mission  and  Results 

1977       Colin,  L.  The  Exploration  of  Venus— Pioneer  Venus  Program  History.  Space  Science  Reviews, 
vol.  20,  May  1977,  pp.  249-258. 

Space  Science  Reviews,  vol.  20,  nos.  3-4,  May-June  1977.  305 

1979       Colin,  L.:  Encounter  with  Venus.  Science,  vol.  203,  no.  4382,  Feb.  23,  1979,  pp.  743-745. 
Science,  vol.  203,  no.  4382,  Feb.  23,  1979. 

Donahue,  T.  M.:  Pioneer  Venus  Results:  An  Overview.  Science,  vol.  205,  no.  4401, 
July  6,  1979,  pp.  41-44. 

Colin,  L.:  Encounter  with  Venus:  An  Update.  Science,  vol.  205,  no.  4401,  July  6,  1979,  pp.  44-46. 

Brodsky,  R.  F.,  ed.:  Pioneer  Venus  Case  Study  in  Spacecraft  Design.  American  Institute  of 
Aeronautics  and  Astronautics,  Inc.,  1979. 

1980  Colin,  L.:  Pioneer  Venus  Overview.  IEEE  Trans.  Geosci.  Remote  Sens.,  vol.  GE-18,  no.  1, 
Jan.  1980,  pp.  3-4. 

IEEE  Transactions.  Geoscience  and  Remote  Sensing,  vol.  GE-18,  no.  1,  Jan.  1980. 

Mutch,  T.  A.:  Introduction  to  Pioneer  Venus.  Special  Issue,  J.  Geophys.  Res.,  vol.  85,  no. 
A13,  Dec.  30,  1980,  p.  7573. 

Colin,  L.:  The  Pioneeer  Venus  Program.  J.  Geophys.  Res.,  vol.  85,  no.  A13,  Dec.  30,  1980, 
pp.  7575-7598. 

Dorfman,  S.  D.;  and  Meredith,  C.  M.:  The  Pioneer  Venus  Spacecraft  Program.  Acta 
Astronaut.,  vol.  7,  no.  6,  1980,  pp.  773-795. 

1981  Evans,  W.  D.,  et  al.:  Gamma-Burst  Observations  from  the  Pioneer  Venus  Orbiter. 
Astrophys.  Space  Sci.,  vol.  75,  no.  1,  Mar.  1981,  pp.  35-46. 

Head,  J.  W.;  Yuter,  S.  E.;  and  Solomon,  S.  C.:  Topography  of  Venus  and  Earth:  A  Test  for 
the  Presence  of  Plate  Tectonics.  Amer.  Sci.,  vol.  69,  no.  6,  1981,  pp.  614-623. 

Hunten,  D.  M.:  The  Upper  Atmosphere  of  Venus:  Implications  for  Aeronomy.  Advances 
in  Space  Research,  vol.  1,  no.  9,  1981,  pp.  3-4. 

Limay,  S.  S.;  and  Suomi,  V.  E.:  Cloud  Motions  on  Venus:  Global  Structure  and  Organiza 
tion.  J.  Atmospheric  Sci.,  vol.  38,  no.  6,  1981,  pp.  1220-1235. 

Masursky,  H.;  Dial,  A.  L.;  Schaber,  G.  G.;  and  Strobell,  M.  E.:  Venus,  a  First  Geologic  Map 
Based  on  Radar  Altimetric  and  Image  Data.  Proc.  Lunar  Sci.  Conf.,  vol.  XII,  March  1981, 
pp.  661-663. 

McGill,  G.  E.;  Steenstrup,  S.  J.;  Barton,  C.;  and  Ford,  P.  G.:  Continental  Rifting  and  the 
Origin  of  Beta  Regio,  Venus.  Geophys.  Res.  Lett.,  vol.  8,  no.  7,  July  1981,  pp.  737-740. 

Morrison,  N.  D.;  and  Morrison,  D.:  The  Surface  of  Venus.  Mercury,  vol.  10,  no.  1,  Jan.- 
Feb.,  1981,  pp.  19-20. 

Nozette,  S.;  and  Ford,  P.:  A  World  Revealed:  Venus  by  Radar.  CAST,  vol.  9,  no.  3,  1981, 
pp.  6-15. 

Phillips,  R.  J.;  Kaula,  W.  M.;  McGill,  G.  E.;  and  Malin,  M.  C.:  Tectonics  and  Evolution  of 
Venus.  Science,  vol.  212,  no.  4497,  1981,  pp.  879-887. 

Reasenberg,  R.  D.;  Goldberg,  Z.  M.;  MacNeil,  P.  E.;  and  Shapiro,  I.:  Venus  Gravity— 

A  High-Resolution  Map.  J.  Geophys.  Res.,  vol.  86,  no.  38,  Aug.  10,  1981,  pp.  7173-7179. 

Russell,  C.  T.;  Luhmann,  J.  G.;  Elphie,  R.  C.;  and  Scarf,  F.  L.:  The  Distant  Bow  Shock  and 
Magnetotail  of  Venus:  Magnetic  Field  and  Plasma  Wave  Observations.  Geophysi.  Res. 
Lett.,  vol.  8,  no.  7,  1981,  pp.  843-846. 

Schubert,  G.;  and  Covey,  C.:  The  Atmosphere  of  Venus.  Sci.  Am.,  vol.  245,  no.  1,  1981,  pp.  66-74. 

Taylor,  F.  W.;  Elson,  L.  S.;  McCleese,  D.  J.;  and  Diner,  D.  J.:  Comparative  Aspects  of  Venus 
and  Terrestrial  Meteorology.  WTHRA,  vol.  36,  no.  2,  1981,  pp.  34-40. 

Watson,  A.  J.;  Donahue,  T.  M.;  and  Walker,  J.  C.  G.:  The  Dynamics  of  a  Rapidly  Escaping 
Atmosphere:  Applications  to  the  Evolution  of  Earth  and  Venus.  Icarus,  vol.  48,  no.  2, 
Nov.  1981,  pp.  150-166. 

1982  Beatty,  J.  K.:  Venus:  The  Mystery  Continues.  Sky  and  Telesc.,  vol.  63,  no.  2,  1982,  pp.  134-138. 
Beatty,  J.  K.:  Report  from  a  Torrid  Planet.  Sky  and  Telesc.,  vol.  63,  no.  5,  1982,  pp.  452-453. 
Campbell,  P.:  After  Pioneer— Good  Science,  Bad  News.  Nature,  vol.  296,  no.  5852,  1982,  pp.  13-14. 

Donahue,  T.  M.;  Hoffman,  J.  H.;  Hodges,  R.  R.,  Jr.;  and  Watson,  A.  J.:  Venus  Was  Wet:  A  Measure- 
ment of  the  Ratio  of  Deuterium  to  Hydrogen.  Science,  vol.  216,  no.  4546,  1982,  pp.  630-633. 

Gold,  M.:  Through  the  Veil  of  Venus.  Science82,  vol.  3,  no.  1,  1982,  p.  12. 

Kliore,  A.  J.;  and  Patel,  I.  R.:  Thermal  Structure  of  the  Atmosphere  of  Venus  from  Pioneer  Venus 
Radio  Occultations.  Icarus,  vol.  52,  no.  2,  Nov.  1982,  pp.  320-334. 

Limaye,  S.  S.:  Polarization  Features  on  Venus.  Bull.  Am.  Astron.  Soe,  vol.  14,  1982,  pp.  740-741. 

Nagy,  A.  F.;  and  Brace,  L.  H.:  Structure  and  Dynamics  of  the  Ionosphere.  Nature,  vol.  296, 
no.  5852,  1982,  p.  19. 

Seiff,  A.;  and  Kirk,  D.  B.:  Structure  of  the  Venus  Mesosphere  and  Lower  Thermosphere  from 
Measurements  during  Entry  of  the  Pioneer  Venus  Probes.  Icarus,  vol.  49,  no.  1,  Jan.  1982, 
pp.  49-70. 

Toon,  O.  B.;  Turco,  R.  P.;  and  Pollack,  J.  B.:  The  Ultraviolet  Absorber  on  Venus:  Amorphous 
Sulfur.  Icarus,  vol.  51,  no.  2,  Aug.  1982,  pp.  358-373. 

1983  Phillips,  R.  J.:  Plate  Tectonics  on  Venus?  Nature,  vol.  302,  no.  5910,  1983,  pp.  655-656. 

Schofield,  J.  T.;  and  Diner,  D.  J.:  Rotation  of  Venus's  Polar  Dipole.  Nature,  vol.  305,  no. 
5930,  Sept.  8,  1983,  pp.  116-119. 

Williams,  B.  G.;  Mottinger,  N.  A.;  and  Panagiotacopulos,  N.  D.:  Venus  Gravity  Field: 
Pioneer  Venus  Orbiter  Navigation  Results.  Icarus,  vol.  56,  no.  3,  Dec.  1983,  pp.  578-589. 

1987  McCormick,  P.  T.;  Whitten,  R.  C.;  and  Knudsen,  W.  C.:  Dynamics  of  the  Venus  lono 
sphere  Revisited.  Icarus,  vol.  70,  no.  3,  1987,  pp.  469-475. 

Phillips,  J.  L.;  and  Russell,  C.  T.:  Upper  Limit  on  the  Intrinsic  Magnetic  Field  of  Venus. 
J.  Geophys.  Res.,  vol.  92,  no.  A3,  1987,  pp.  2253-2263. 

Scarf,  F.  L.;  Jordan,  K.  F.;  and  Russell,  C.  T.:  Distribution  of  Whistler  Mode  Bursts  at  Venus. 
J.  Geophys.  Res.,  vol.  92,  no.  All,  1987,  pp.  12,407-12,411. 

Stewart,  A.  I.  F.:  Pioneer  Venus  Measurements  of  H,  O,  and  C  Production  in  Comet  P/ 
Halley  near  Perihelion.  Astron.  Astrophys.,  vol.  187,  nos.  1-2,  1987,  pp.  369-374. 

Taylor,  H.  A.,  Jr.;  Cloutier,  P.  A.;  and  Zheng,  Z.:  Venus  "Lightning"  Signals  Reinterpreted  as 
In  Situ  Plasma  Noise.  J.  Geophys.  Res.,  vol.  92,  no.  A9,  1987,  pp.  9907-9919. 

1988  Brace,  L.  H.;  Theis,  R.  F.;  Curtis,  S.  A.;  and  Parker,  L.  W.:  A  Precursor  to  the  Venus  Bow 
Shock.  J.  Geophys.  Res.,  vol.  93,  no.  All,  1988,  pp.  12,735-12,749. 

Fenimore,  E.  E.;  et  al.:  Interpretations  of  Multiple  Absorption  Features  in  a  Gamma-Ray 
Burst  Spectrum,  Part  II.  Astrophys.  J.,  vol.  335,  1988,  pp.  L71-L74. 

Kasting,  J.  F.;  Toon,  O.  B.;  and  Pollack,  J.  B.:  How  Climate  Evolved  on  the  Terrestrial 
Planets.  Sci.  Amer.,  vol.  258,  Feb.  1988,  pp.  90-97. 

Knudsen,  W.  C.:  Solar  Cycle  Changes  in  the  Morphology  of  the  Venus  Ionosphere. 
J.  Geophys.  Res.,  vol.  93,  no.  A8,  1988,  pp.  8756-8762. 

Mayr,  H.  G.;  Harris,  I.;  and  Kasprzak,  W.  T.:  Gravity  Waves  in  the  Upper  Atmosphere  of 
Venus.  J.  Geophys.  Res.,  vol.  93,  no.  A10,  1988,  pp.  11,247-11,262. 

Montoya,  E.  J.;  and  Fimmel,  R.  O.:  Pioneers  in  Space:  The  Story  of  the  Pioneer  Missions 
(Part  1).  Mercury,  vol.  17,  no.  2,  Mar.-Apr.  1988,  pp.  56-62. 

Montoya,  E.  J.;  and  Fimmel,  R.  O.:  Pioneers  in  Space:  The  Story  of  the  Pioneer  Missions 
(Part  2).  Mercury,  vol.  17,  no.  3,  May-June  1988,  pp.  81-89. 

Scarf,  F.  L.;  and  Russell,  C.  T.:  Evidence  of  Lightning  and  Volcanic  Activity  on  Venus; 
Pro  and  Con.  Science,  vol.  240,  no.  4849,  1988,  pp.  222-224. 

1989       Kim,  J.;  Nagy,  A.  F.;  Cravens,  T.  E.;  and  Kliore,  A.  J.:  Solar  Cycle  Variations  of  the  Elec 

tronic  Densities  Near  the  Ionospheric  Peak  of  Venus.  J.  Geophys.  Res.,  vol.  94,  no.  A9,1989, 
pp.  11,997-12,002. 

Slavin,  J.  A.;  Intriligator,  D.  S.;  and  Smith,  E.  J.:  Pioneer  Venus  Orbiter  Magnetic  Field  and 
Plasma  Observations  in  the  Venus  Magnetotail.  J.  Geophys.  Res.,  vol.  94,  no.  A3,  1989, 
pp.  2383-2398. 

Woo,  R.;  et  al.:  Solar  Wind  Interaction  with  the  Ionosphere  of  Venus  Inferred  from  Radio 
Scintillation  Measurements.].  Geophys.  Res.,  vol.  94,  no.  2,  1989,  pp.  1473-1478. 

Russell,  C.  T.,  ed.:  Venus  Aeronomy.  Space  Sci.  Rev.,  vol.  55,  nos.  1-4,  Jan.-Feb.  1991,  pp.  1-489. 

Strangeway,  R.  J.:  The  Pioneer  Venus  Orbiter  Entry  Phase.  Geophys.  Res.  Lett.,  vol.  20,  no.  23, 

1994,  pp.  2715-2717. 

Luhmann,  J.  G.;  Pollack,  J.  B.;  and  Colin,  L:  The  Pioneer  Mission  to  Venus.  Sci.  Amer.,  vol.  270, 
no.  4,  April  1994,  pp.  90-97. 

Venus  Exploration:  Background  and  Other  Missions 

1963       Newlan,  I.:  First  to  Venus:  The  Story  of  Mariner  II.  McGraw-Hill,  1963. 
1965       Mariner-Venus  1962:  Final  Project  Report.  NASA  SP-59,  1965. 
1971       Mariner-Venus  1967:  Final  Project  Report.  NASA  SP-190,  1971. 
1975       Hansen,  J.  E.,  ed.:  The  Atmosphere  of  Venus.  NASA  SP-382,  1975. 

Conference  on  the  Atmosphere  of  Venus.  J.  Atmos.  Sci.,  vol.  32,  no.  6,  1975,  pp.  1005-1265. 

1977  Murray,  B.;  and  Burgess,  E.:  Flight  to  Mercury.  Columbia  Univ.  Press,  1977.  309 

1978  Dunne,  J.  A.;  and  Burgess,  E.:  The  Voyage  of  Mariner  10:  Missions  to  Venus  and  Mercury. 
NASA  SP-424,  1978. 

1979  Johnson,  N.  L.:  Handbook  of  Soviet  Lunar  and  Planetary  Exploration.  Science  &  Tech.  Series, 
American  Astronautical  Soc.,  Univelt,  Inc.,  vol.  47,  1979. 

1981       Fichtel,  C.  E.;  and  Trombka,  J.  L:  Gamma  Ray  Astrophysics:  New  Insight  Into  the  Universe. 
NASASP-453,  1981. 

1982  Burgess,  E.:  Venus:  The  Twin  that  Went  Wrong.  New  Sci.,  vol.  94,  no.  1310,  June  17, 
1982,  pp.  786-789. 

1983  Hunten,  D.  M.;  et  al.,  eds.:  Venus.  Univ.  of  Arizona  Press,  1983. 

Fimmel,  R.  O.;  Colin,  L.;  and  Burgess,  E.:  Pioneer  Venus.  NASA  SP-461,  1983. 

1985  Burgess,  E.:  Venus:  An  Errant  Twin.  Columbia  Univ.  Press,  1985. 

1986  Byford,  S.:  Venus  Unveiled  by  Vega  Probes.  SPFLA,  vol.  28,  no.  2,  Feb.  1986,  pp.  61-62. 

1992       Robertson,  D.  F.:  Venus— A  Prime  Soviet  Objective.  SPFLA,  vol.  34,  no.  5,  May  1992,  pp. 

Robertson,  D.  F.:  Venus— A  Prime  Soviet  Objective.  SPFLA,  vol.  34,  no.  6,  June  1992, 
pp. 202-205. 

Saunders,  R.  S.;  et  al.:  Magellan  Mission  Summary.  J.  Geophys.  Res.,  vol.  97,  no.  E8,  Aug.  25, 
1992,  pp.  13,067-13,090. 

Magellan  at  Venus.  J.  Geophys.  Res.,  vol.  97,  no.  E8,  Aug.  25,  1992,  pp.  13,062-13,689. 




Abbreviations  of  science  instruments,  list  of,  table  2-2  (p.  29) 
Academy  of  Sciences  (U.S.S.R.) 

Institute  of  Earth  Magnetism  and  Radiowave  Propagation,  252 

Institute  of  Space  Research,  252 
Acceleration  sensors,  115 

ACS  (attitude  control  system)  telemetry  format,  62 
Aeronomy  experiments,  Orbiter,  135 
Aeroshell  of  Small  Probes,  76 

Airglow  from  UV  absorption  by  atmospheric  gases,  88 
Airglows  in  Venusian  atmosphere,  217 
Airglow  ultraviolet  spectrometer,  88-90 

in  comet  observations,  90 

in  measuring  UV  light  from  Venusian  clouds,  88 

operational  modes  of,  90 

photomultipliers  on,  89-90 

in  studying  absence  of  water  on  Venus,  88-89 
Akna  Montes  mountain  range,  176 
Alpha  Regio  plateau,  description  of,  1 79 

Aluminum  equipment  shelves  as  heat  sinks  on  Small  Probe,  140 
Aluminum-26  effects  on  planetary  volatiles,  212 
Ames  Research  Center,  32,  33,  42,  92,  108,  111,  115,  138,  148,  155,  157,  158,  246 

development  of  Venus-mission-test  spacecraft,  23-24 

40-  by  80-Foot  Wind  Tunnel,  in  tests  of  Multiprobe  parachute  system,  42 

transfer  of  Venus  mission  to,  23 
Amplifiers,  Orbiter  spacecraft,  performance  of,  162 

Analytical  instruments  on  Venera  spacecraft.  See  Soviet  science  instruments 

charged-particle  retarding  potential.  See  Charged-  particle  retarding  potential  analyzer 

retarding  potential,  on  Orbiter,  82 

solar  wind  plasma.  See  Solar  wind  plasma  analyzer 
Anderson,  C.  E.,  169 
Announcement  of  Opportunity.  See  AO 
Anorthosite-troctolite  on  Venus,  271 
Antenna  3  -^  ^ 

despun,  for  Orbiter,  63 

Large  Probe,  72 

omnidirectional,  for  Multiprobe  spacecraft,  68 

parabolic  dish,  for  Orbiter,  57 
Antennas  for  Small  Probes,  77 
Antenna  systems 


despun,  63 

features  of,  63 

operating  characteristics  of,  63-65 

Anviloy  seals,  139 

AO  (Announcement  of  Opportunity) 

for  Multiprobe  mission,  28 

for  Orbiter  mission,  28 

for  scientists  to  participate  in  Venus  program,  23 
APOA  telemetry  format,  62 
APOB  telemetry  format,  62 
Aphrodite  Terra,  175-176,  181 

as  landing  site  of  Vega  spacecraft,  270 

description  of,  177-178 

height  above  mean  radius,  1 76 
Apoapsis,  35 

definition  of,  27 

high  concentrations  of,  in  Venusian  atmosphere,  144 

isotope  ratios,  245,  248,  267 

in  Venusian  atmosphere,  211,  245-246,  267 

radiogenic  isotopes  of,  245-246 
Argon-36,  245,  267 

in  Venusian  atmosphere,  211,  212 
Argon-40,  in  Venusian  atmosphere,  212,  245,  267 
Argon-to-krypton  ratios,  212 
Arizona,  University  of,  33,  109 
Army  Ballistic  Missile  Agency,  17 
Artemis  Chasma,  178 
Asteria  Regio,  1 78 
Atalanta  Planitia  basin,  175 
Atla  Regio,  176 
Atlas-Centaur,  50 

launch  of  Pioneer  Venus 

Multiprobe  spacecraft,  51 
Orbiter  spacecraft,  50 

launch  vehicle 

description  of,  39 
for  Pioneer  Venus,  39 
propellants  for,  39 

Atlas  SLV-3D  booster.  See  Atlas-Centaur  launch  vehicle 
Atmosphere.  See  also  Venusian  atmosphere 


composition  of,  7-8 
diagram  of,  figure  1-9  (p.  10) 

drag  experiment,  103-104 

propagation  experiment,  Multiprobe,  117 


experiment  on  Large  Probe,  72,  113-115 
sensor  boom,  failure  of,  145-146 
turbulence,  103 

experiment,  Multiprobe,  119 

Atmospheric  and  solar-wind  turbulence  experiment,  103 
Atomic  oxygen,  190 

as  factor  in  atmospheric  cooling,  189 
response  of,  to  solar  activity,  190 
Attitude  control  system.  See   ACS 
Auroras  in  Venusian  atmosphere,  217 
AVCO  Corporation 

study  contract  for  Venus  probe  mission,  21 
Venus  study  contract,  1 7 

Babakin,  G.  N.,  in  design  of  Venera  spacecraft,  241 
Ball  Brothers  Research,  32,  33 
Balloons  as  atmospheric  probes,  19 
Barnes,  A.,  29 

and  Orbiter  solar  wind  experiment,  92-93 

Orbiter,  degradation  of,  162-163 

for  Small  Probes,  78 
Bauer,  S.,  30,  120,  261 

Bearing  and  power  transfer  assembly,  Orbiter,  59 
Bendix  Field  Engineering  Corporation,  46,  158 
Beryllium  equipment  shelves  as  heat  sinks  on  Large  Probe,  140 
BetaRegio,  175-176,  177,  181 

decription  of,  178-179 
Bit  flips,  130-131 

in  adversely  affecting  spacecraft  timing  sequences,  131 

influence  of  cosmic  rays  on,  131 

on  Orbiter  spacecraft,  131 
Blamont,  J.,  and  nephelometer  experiment,  115 
Boese,  R.,  29 

and  infrared  radiometer,  111 
Bonn,  University  of,  104 
Book,  Orange,  24 
Book,  Purple,  21,  22 
Bowin,  C.  O.,  121 
Bow  shock  wave,  Venusian,  asymmetry  of,  254.  See  also    Solar  wind 



Brace,  L.,  29 

and  electron-temperature  probe,  96 
and  Orbiter  Entry  Science  Plan,  155 
Breus,  T.,  259,  262,  263 
Brinton,  H.,  28 

British  Aerospace  Company,  28 
British  Royal  Observatory  at  Greenwich,  169 

in  atmospheric  measurements,  210 

measurements  of  helium  in  upper  Venusian  atmosphere,  213 

Multiprobe.  See  also  Multiprobe  Bus 

entry  of,  into  Venusian  atmosphere,  143-144 
experiments  on,  83 
scientific  instruments  on,  83 
for  Multiprobe  spacecraft,  66-68 

major  assemblies  of,  66 
Butler,  D.,  263 

California,  University  of,  at  Los  Angeles,  94,  95,  155 
Canberra  DSN  facility,  48 
Carbon  dioxide 

as  most  abundant  gas  in  Venusian  atmosphere,  243 

in  Earth  carbonates,  248 

in  Earth  rocks,  276 

effects  on  radiative  properties  and  dynamics  of  Venus's  atmosphere,  8 

factors  affecting  atmospheric  concentrations  of,  210 

in  Venusian  atmosphere,  7-8,  210,  214,  248,  276. 

See  also  Venusian  atmosphere 
Carbon  monoxide  in  Venusian  atmosphere,  213 
Carbonyl  sulfide,  214 
Celestial  mechanics 

experiment,  102 

in  navigating  Pioneer  Venus  spacecraft,  127 
Cells  in  Venusian  atmosphere,  188 
Ceramic  microleak  (CML)  inlet,  31-32 
Chamberlain,].,  18,263 
Charged-particle  retarding  potential  analyzer,  97-100 

in  detection  of  ionospheric  plasma  particles,  99 

ionospheric  measurements  with,  97 

operation  of,  99-100 

particles  in  Venusian  clouds,  251 


role  of,  in  cloud  chemistry,  200 
Chromatograph,  gas.  See  Gas  chromatograph 
Circumequatorial  belts  in  Venusian  atmosphere,  188 

caps  as  haze  particles,  186-187 
layers  on  Venus 

physical  properties  of,  table  6-2  (p.  202) 

thickness  of,  1 1 

in  infrared,  193 

in  ultraviolet,  193 

in  visible  light,  193 
particles,  198-202 

as  sulfur  droplets,  193 

composition  of,  200 

determining  size  of,  271 

influence  of  circulation  on  growth  of,  205 

size  distributions  of,  193,  198-200 
Cloud  particle-size  spectrometer,  32-33.  See  also  under  Mass  spectrometer  headings 

on  Large  Probe,  113 
Cloud  photopolarimeter,  84-85 
description  of,  84-85 
fields  of  view  of,  85 
observations  in  visible  light  with,  85 
purpose  of,  84 

in  studies  of  Venusian  haze,  85 
telescope,  observation  angles  of,  85 
Cloud  polarirneter,  182-183 
Clouds.  See  also   Venusian  atmosphere 
absence  of  information  about,  26 
absorption  characteristics  of,  200-202 
analyses  of  composition  of,  271 
as  lightning  sources,  207 

atmospheric,  193-198  71  <; 

basic  chemistry  of,  205 
chlorine  particles  in,  251 
composition  of,  250-251 
constituents  of,  9-11 

features  of,  observed  in  ultraviolet,  195-196 
general  nature  of,  193 
instruments  for  study  of,  83-84 
lower  region,  variability  of,  195 
measurement  of  particle  sizes  in,  84 

Clouds,  (continued) 

Orbiter  study  of,  82 

Pioneer  Venus  instruments  for  study  of,  194 
principal  la